[SPIR-V] Validation Error: ID decorated with RelaxedPrecision multiple times is not allowed.

[NicVanZuylen]

Description
Compiling HLSL which implements relaxed-precision types to SPIR-V may fail with a SPIR-V validation error similar to the following:
ID '[SPIR-V Result ID]' decorated with RelaxedPrecision multiple times is not allowed.

This issue may be caused by optimizations around relaxed-precision arithmetic operations, possibly in tandem with loop unrolling optimizations.

When disabling optimizations and/or relaxed-precision types [Remove] -DFB_SHADERCOMPILER_16BITRELAXEDPRECISION=1, -Od, the issue will not be present.

Steps to Reproduce

1. Checkout and build DXC at SHA: 99400dc ('main' branch)
2. Run the built dxc.exe on the following provided HLSL source file in:
   problemhlsl.zip
   with arguments:
   -T cs_6_0 -E csMipBatchBlur problemhlsl.hlsl -Fo problemshader.spv -HV 2018 -spirv -O3 -DFB_SHADERCOMPILER_16BITRELAXEDPRECISION=1
3. Observe output for the aforementioned error.
4. Try with optimizations and/or relaxed-precision disabled [Remove] -DFB_SHADERCOMPILER_16BITRELAXEDPRECISION=1, -Od and observe that the shader will compile to SPIR-V successfully.

Actual Behavior
Running dxc.exe with the above reproduction steps will produce the following error:

fatal error: generated SPIR-V is invalid: ID '9' decorated with RelaxedPrecision multiple times is not allowed.
  %9 = OpFAdd %float %41 %42

note: please file a bug report on https://github.com/Microsoft/DirectXShaderCompiler/issues with source code if possible

Environment

• DXC version: dxcompiler.dll: 1.10(5371-99400dc8)(1.10.2605.0) - 1.10.2605.0
• Host Operating System: Windows 11 24H2 Build 26100.7462
• Commit SHA: 99400dc80ffa0eb6aef56fc6e482f5eff40185e4