[llvm] [SPIRV] Preserve sign-sensitive width record across G_TRUNC replacement (PR #211626)

[Faijul Amin via llvm-commits]

mdfaijul wrote:

@MrSidims I missed a case when G_TRUNC gets removed in the `SPIRVPreLegalizer` due to same new width for `dst` and `src`. This PR fixes this.
```
  %a.i4 = trunc i8 %a to i4
  %b.i4 = trunc i8 %b to i4
  %cmp = icmp slt i4 %a.i4, %b.i4
  ret i1 %cm
  ```

https://github.com/llvm/llvm-project/pull/211626