[SPIR-V] -Gis causes multiple NoContraction

Description
Compiling certain inputs to SPIRV with the flag -Gis causes a compile error.

Steps to Reproduce
The following link demonstrates a trivial input that has this behaviour.
https://godbolt.org/z/rq6r1j7on

Actual Behavior
https://godbolt.org/z/rq6r1j7on

Compiling with the flags -spirv -Gis causes a compile error with the following output:
fatal error: generated SPIR-V is invalid: ID '8' decorated with NoContraction multiple times is not allowed.
  %8 = OpIMul %v2uint %31 %13

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

https://godbolt.org/z/q9cMnP7jh
Compiling with the flags -Vd on top removes the validation step and doesn't cause the error. However, the generated SPIRV code contains multiple NoContraction instructions:

 OpDecorate %9 NoContraction
 OpDecorate %9 NoContraction

This corresponds with the error message produced without -Vd.

Environment

• DXC version: dxcompiler.dll: 1.9 - 1.8.2505.1
• Host Operating System: Windows 11 Enterprise version 24H2

Start with the Godbolt reproducer at https://godbolt.org/z/rq6r1j7on and compare compilation with -spirv -Gis, with and without -Vd. Trace the SPIR-V generation path responsible for NoContraction decorations and inspect the generated output shown in the issue. Done means the reproducer no longer emits duplicate decorations and validation succeeds.