[MLIR][SPIRVToLLVM] Removed std to llvm patterns from the conversion

Removed the Standard to LLVM conversion patterns that were previously
pulled in for testing purposes. This helps to separate the conversion
to LLVM dialect of the MLIR module with both SPIR-V and Standard
dialects in it (particularly helpful for SPIR-V cpu runner). Also,
tests were changed accordingly.

Reviewed By: mravishankar

Differential Revision: https://reviews.llvm.org/D86285