[AMDGPU] Expand DAG Test Coverage for i16 -> i32 uniform promotion

[Pierre-vh]

#109942 fixed a bug, but no tests were updated. This hints at a lack of test coverage.

[llvmbot]

@llvm/issue-subscribers-backend-amdgpu

Author: Pierre van Houtryve (Pierre-vh)

#109942 fixed a bug, but no tests were updated. This hints at a lack of test coverage.

[Pierre-vh]

This is now covered as of #140208 - lots of tests fail if the typo in that commit isn't fixed (so if SHL isn't handled)