IRToASTVisitor: handle fixed-vector insertion, extraction and shuffle #350

A valid fixed-width LLVM insertelement instruction is rejected before C
emission. extractelement and shufflevector also lack instruction visitors.

Standalone synthetic example:

define i64 @byte_insert_constant(i64 %bits, i8 %value) {
  %v = bitcast i64 %bits to <8 x i8>
  %updated = insertelement <8 x i8> %v, i8 %value, i32 3
  %result = bitcast <8 x i8> %updated to i64
  ret i64 %result
}

rellic-decomp --unstructured --input vector-lane-module.ll --output vector-lane-module.c
clang --target=i686-pc-windows-msvc -std=c2x -O2 -c vector-lane-module.ll -o original.obj
clang --target=i686-pc-windows-msvc -std=c2x -O2 -c vector-lane-module.c -o rebuilt.obj

Observed failure: Instruction not supported: insertelement <8 x i8>.
Expected: recompilable C preserving the original input vector, selected
lane, shuffle indices and exact memory-access byte counts.

The generic implementation copies the input vector into a temporary for
insertion, uses vector subscripting for extraction, and emits
__builtin_shufflevector for shuffle masks. Vector operations with align 1
need byte-count compiler-builtin memory copies instead of naturally aligned
C vector-pointer dereferences; otherwise recompilation can replace
movups/movdqu with alignment-requiring movaps/movdqa.

The attached full synthetic module has 14 scalar/pointer-interface exports,
covering constant and masked dynamic indices, byte-vector bitcasts,
reverse/splat/two-input shuffles, different result lengths and unaligned
vector loads/stores. Its whole-DLL comparison covers every export. Six bodies
are byte-identical; seven pairs contain identical instructions reordered;
one uses a different SIMD instruction selection. Those nonexact differences
remain visible and are not presented as semantic-equivalence proof.

Tested at public fork c2b4695dd220ab4249914a8d4d52556e036e9f08
with LLVM/Clang 17.0.6. Upstream 370abaad lacks the three instruction visitors
by source inspection; a clean upstream runtime build is not claimed.
Historical PR #175 added vector types/constants, not these lane visitors.
No exact duplicate was found in the repository issue/PR search.

Standalone full module: https://github.com/humdrum00001010/rellic/blob/054477b0cd245ef0b0e72e4955f674013ff5cb2c/tests/tools/decomp/ir/vector-lane-module.ll

Generic fix: https://github.com/humdrum00001010/rellic/commit/054477b0cd245ef0b0e72e4955f674013ff5cb2c