[mlir] Fix a warning

This patch fixes:

  mlir/lib/Conversion/AMDGPUToROCDL/AMDGPUToROCDL.cpp:128:10: warning:
  variable ‘llvm2xI32’ set but not used [-Wunused-but-set-variable]

The last use of llvm2xI32 was removed on July 6, 2022 in commit
63295622.