[SPIR-V] Fix sub-byte argument type mismatch in call lowering (#189893)
Remove the `MRI->setType()` call in lowerFormalArguments that was
overriding the vreg LLT with the SPIR-V type's LLT, causing a type
mismatch when sub-byte integer arguments (e.g. i2, i4) were preliminary
widened to i8
related to https://github.com/llvm/llvm-project/issues/188703