[SPIR-V] fatal error: generated SPIR-V is invalid: ID decorated with RelaxedPrecision multiple times is not allowed.

[castano]

Description

Loop unrolling causes duplicate decoration when the phi instruction is decorated. This is due to a bug in spirv-opt: KhronosGroup/SPIRV-Tools#6689

I've already provided a fix for the bug in spirv-tools: KhronosGroup/SPIRV-Tools#6690

But this will need to merged once the issue is fixed.

Steps to Reproduce

This is a simple shader that reproduces the issue:

RWStructuredBuffer<float> outp;
StructuredBuffer<float> inp;
[numthreads(1, 1, 1)]
void main() {
    min16float acc = 0;
    [unroll] for (uint i = 0; i < 8; i++) {
        min16float x = (min16float)inp[i];
        acc = min(acc, x * x);
    }
    outp[0] = acc;
}

You can check it out here: https://godbolt.org/z/eT7dr797f

Actual Behavior
The compiler produces this output:

fatal error: generated SPIR-V is invalid: ID '11' decorated with RelaxedPrecision multiple times is not allowed.
  %11 = OpExtInst %float %1 NMin %23 %24

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

Compiler returned: 5

Environment
The issue occurs in dxc 1.10 and 1.9, but not in 1.8