Bad alignment generated for loads/stores from half arrays

Description

When compiling shaders from FidelityFX-SPD, DXC generates load and store instructions with bad alignment for arrays of type half.

In the DXIL below, the half array is aligned to 4 bytes and so are the loads and stores, even the ones with odd indices which are only guaranteed to be aligned to 2 bytes.

  %10 = alloca [4 x half], align 4
  %11 = alloca [4 x half], align 4
  %12 = alloca [4 x half], align 4
  %13 = alloca [4 x half], align 4
...
  %140 = getelementptr [4 x half], [4 x half]* %10, i32 0, i32 1
  %141 = getelementptr [4 x half], [4 x half]* %11, i32 0, i32 1
  %142 = getelementptr [4 x half], [4 x half]* %12, i32 0, i32 1
  %143 = getelementptr [4 x half], [4 x half]* %13, i32 0, i32 1
  store half %136, half* %140, align 4
  store half %137, half* %141, align 4
  store half %138, half* %142, align 4
  store half %139, half* %143, align 4
...
  %431 = load half, half* %140, align 4
  %432 = load half, half* %141, align 4
  %433 = load half, half* %142, align 4
  %434 = load half, half* %143, align 4

Steps to Reproduce

1. Clone https://github.com/GPUOpen-Effects/FidelityFX-SPD
2. Build DX12 project
3. Go to sample\bin\ShaderLibDX
4. Run dxc.exe -T cs_6_3 -E main .\SPDIntegrationLinearSampler.hlsl -DA_HALF=1 -DSPD_PACKED_ONLY=1 -DSPD_NO_WAVE_OPERATIONS=1

Actual Behavior

Generated DXIL has 4-byte alignment for the half loads/stores.

Environment

• DXC versions tried (all repro):
  • dxcompiler.dll: 1.9 - 1.8.2505.28 (9efbb6c32); dxil.dll: 1.9(1.8.2505.28)
  • dxcompiler.dll: 1.6 - 1.5.0.2743 (1db4cd60); dxil.dll: 1.3(10.0.17763.132)
• Host Operating System: Win11 26100

Reproduce the issue with the listed FidelityFX-SPD shader and the dxc.exe command, then trace how half arrays and their load/store instructions receive alignment during compilation. Done means the generated DXIL uses alignment no greater than 2 bytes for odd-index half accesses while preserving valid alignment elsewhere.