[SPIRV][HLSL] Add FixedVector GEP legalization (#171682)

fixes #170241

PR #169090 updated vector swizzle elements individually for HLSL This is
because rawbuffer writes via rawBufferStore can cause datarace in
concurrent writing the way vectors are written all at once. This means
we needed individual writes per element. So that means we need to be
able to GEP into an element of a vector.

The SPIRV backend did not support this pattern. SPIRV assumes Composite
types (ie Vectors, Structs, and Arrays) only for Ptr legalization in
it's store transformations via transformStore.

Fixing things at the point of ptr legalziation for the store would be
too late because we would have lost critical ptr type information still
available in LLVM IR. Instead what we needed to do is teach the
walkLogicalAccessChain used by buildLogicalAccessChainFromGEP to
converts a byte-offset GEP on an i8-pointer into a logical SPIR-V
composite access chain.

In short:

The fundamental issue is that walkLogicalAccessChain currently refuses
to handle vector-based logical GEPs, but Clang’s new HLSL path produces
exactly that pattern:
- you have a logical i8* GEP that represents indexing into the elements
of a <4 x i32>.

We need to teach walkLogicalAccessChain to treat a FixedVectorType
similarly to an array of its element type. So in walkLogicalAccessChain
replace the “give up on vector” part with logic that:
- Interprets the byte Offset as indexing into the vector elements.
- Uses the vector’s element type and number of elements.