[MLIR][SPIRVToLLVM] Miscellaneous ops conversion: select, fmul and undef

This patch introduces 3 new direct conversions for SPIR-V ops:
- `spv.Select`
- `spv.Undef`
- `spv.FMul` that was skipped in the patch with arithmetic ops

Differential Revision: https://reviews.llvm.org/D83291