[SPIR-V] `uint64_t2/int64_t2` typed buffers emitted with `Unknown` image format

[kmpeng]

This is the DXC side of llvm/offload-test-suite#1037

Description
DXC's SPIR-V codegen emits an Unknown image format for [RW]Buffer<uint64_t2/int64_t2> because SPIR-V has no multi-component 64-bit image format for the 2-component 64-bit element to lower to. This ultimately generates incorrect outputs in the offload-test-suite (see the discussion in llvm/offload-test-suite#1037 for more explanation on how).

Repro
https://godbolt.org/z/bWshMxheq

RWBuffer<uint64_t2> U0 : register(u0);
RWBuffer<uint64_t> Out0 : register(u1);

[numthreads(4,1,1)]
void main(uint GI : SV_GroupIndex) {
  Out0[GI] = U0[GI].x;
}

OpTypeImage %ulong Buffer 2 0 0 2 Unknown

Requested Fix
DXC should pack 2-component 64-bit typed buffer elements into 4-component 32-bit images and bitcast during load/store. This matches how DirectX already handles these buffers: uint64_t2/int64_t2 elements are backed by 4x32-bit channels and reinterpreted in the shader.

This same fix is being made in Clang: llvm/llvm-project#208626