[SPIR-V] SPIR-V back-end is generating no-op conversions

We're trying to switch the Mesa OpenCL infra over to using the SPIR-V back-end in LLVM and a bunch of OpenCL CTS tests are failing to validate with:
```
(file=input,line=0,column=0,index=59): Expected input to have different bit width from Result Type: UConvert
```

Start by reproducing the OpenCL CTS validation failures while using the SPIR-V back-end, then trace the handling of UConvert values that produces the reported bit-width error. Done means the affected conversions are emitted with valid differing input and result widths and the failing CTS tests validate successfully.

Written by the indexing model from the issue text.