JIT: fix arm32 dump/disasm of references to async resume info (dotnet#123814)

## Description

ARM32 JIT dump/disasm asserts when encountering async resume info
references. The `IF_T2_N2` instruction format handles both block address
tables and async resume info, but the dump logic only expected the
former.

## Changes

- **Remove assert in `emitarm.cpp:7308`**: Was failing when `jdsc` is
null (async resume info case)
- **Add null check before table dump**: Skip inline block address table
dump when `jdsc` is null
- **Use `nullptr` instead of `NULL`**: Modern C++ style throughout the
code block
- **Add conditional printing**: Print `RWD%02zu` format for async resume
info (when `jdsc` is null) and `J_M%03u_DS%02u` format for block address
tables (when `jdsc` is not null)
- **Clarify comment**: Document when `jdsc` is null vs non-null

The fix preserves all existing dump behavior for block address tables
while allowing async resume info to display properly without assertion.

```cpp
// Before: would assert if jdsc is null and always print as DS format
assert(jdsc != NULL);
printf("... J_M%03u_DS%02u", ...);
if (id->idIns() == INS_movt) { /* dump table using jdsc */ }

// After: skip table dump when jdsc is null and use appropriate format
if (jdsc != nullptr)
    printf("... J_M%03u_DS%02u", ...);  // block address table
else
    printf("... RWD%02zu", ...);         // async resume info
if (jdsc != nullptr && id->idIns() == INS_movt) { /* dump table using jdsc */ }
```

Affects dump/disasm output only; no impact on code generation.

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<details>

<summary>Original prompt</summary>

> 
> ----
> 
> *This section details on the original issue you should resolve*
> 
> <issue_title>JIT: fix arm32 dump/disasm of references to async resume
info</issue_title>
> <issue_description>See
dotnet#123781 (comment)
> 
> Seemingly `IF_T2_N2` expects the data to resolve to a table of block
addresses, so if we reuse this form for references to async resume info
it doesn't display properly.
> 
> I think this just impacts dumping and disasm. But not 100% sure.
> </issue_description>
> 
> <agent_instructions>Fix this issue by handling gracefully the
situation where `jdsc` is null -- skip the inline table dump logic in
that case</agent_instructions>
> 
> ## Comments on the Issue (you are @copilot in this section)
> 
> <comments>
> </comments>
> 


</details>



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- Fixes dotnet#123813

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Author: 2 people

src/coreclr/jit/emitarm.cpp

@@ -7285,7 +7285,7 @@ void emitter::emitDispInsHelp(
             emitDispReg(id->idReg1(), attr, true);
             imm = emitGetInsSC(id);
             {
-                dataSection*  jdsc = 0;
+                dataSection*  jdsc = nullptr;
                 NATIVE_OFFSET offs = 0;
 
                 /* Find the appropriate entry in the data section list */
@@ -7305,22 +7305,29 @@ void emitter::emitDispInsHelp(
                     offs += size;
                 }
 
-                assert(jdsc != NULL);
-
                 if (id->idIsDspReloc())
                 {
                     printf("reloc ");
                 }
-                printf("%s ADDRESS J_M%03u_DS%02u", (id->idIns() == INS_movw) ? "LOW" : "HIGH", emitComp->compMethodID,
-                       imm);
 
-                // After the MOVT, dump the table
-                if (id->idIns() == INS_movt)
+                if (jdsc != nullptr)
+                {
+                    printf("%s ADDRESS J_M%03u_DS%02u", (id->idIns() == INS_movw) ? "LOW" : "HIGH",
+                           emitComp->compMethodID, imm);
+                }
+                else
+                {
+                    printf("%s ADDRESS RWD%02zu", (id->idIns() == INS_movw) ? "LOW" : "HIGH", (size_t)imm);
+                }
+
+                // After the MOVT, dump the table if jdsc is not null. jdsc is null for async resume info
+                // and non-null for block address tables.
+                if (jdsc != nullptr && id->idIns() == INS_movt)
                 {
                     unsigned     cnt = jdsc->dsSize / TARGET_POINTER_SIZE;
                     BasicBlock** bbp = (BasicBlock**)jdsc->dsCont;
 
-                    bool isBound = (emitCodeGetCookie(*bbp) != NULL);
+                    bool isBound = (emitCodeGetCookie(*bbp) != nullptr);
 
                     if (isBound)
                     {