JIT: Fold constant access to [ReadOnly]Span<T> especially for char and byte

[EgorBo]

In order to be able to declare unrolling rules in pure C# the way it was proposed in #64821 for strings we need to be able to fold this in JIT:

T SecondChar<T>(ReadOnlySpan<T> span) => span[1];
T GetLength<T>(ReadOnlySpan<T> span) => span.Length;

char Test1() => SecondChar<char>("hello"); // => 'e'
int  Test2() => GetLength<char>("hello"); // => 5

Both Test1 and Test2 has to be folded to constants.
Ideally we should be able to also handle spans of bytes (utf8)

Also, fold constant accesses to RVA data (from #64612). Example:

static ReadOnlySpan<byte> data => new byte[] { 1, 2, 3, 4, 5 };

byte Test() => data[3];

Current codegen:

; Method Prog:Test():ubyte:this
       48B8A32BAD36FD010000 mov      rax, 0x1FD36AD2BA3
       0FB600               movzx    rax, byte  ptr [rax]
       C3                   ret      
; Total bytes of code: 14

Expected codegen:

; Method Prog:Test():ubyte:this
       B804000000           mov      eax, 4
       C3                   ret      
; Total bytes of code: 6

NOTES:

• JIT is already able to do it for string literals e.g. "hello"[2] (see fgMorphArrayIndex)
• C# should soon get a support for ROS<any primitive> = RVA, not just byte/bool as it is currently
• We probably can't do it for RVAs in C++/CLI/CX which are mutable (check for mixed-mode)
• This task most likely needs a new JIT-EE API or maybe the existing getArrayInitializationData is enough

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Issue Details

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In order to be able to declare unrolling rules in pure C# the way it was proposed in #64821 for strings we need to be able to fold this in JIT:

T Bar<T>(ReadOnlySpan<T> span) => span[0];

char Foo1() => Bar<char>("hello");
byte Foo2() => Bar<byte>(new byte[] { 1,2,3 });

Both Foo1 and Foo2 has to be folded to constants.

Author:	EgorBo
Assignees:	-
Labels:	up-for-grabs, area-CodeGen-coreclr, untriaged
Milestone:	-