[MLIR][SPIRVToLLVM] Add skeleton for SPIR-V to LLVM dialect conversion
These commits set up the skeleton for SPIR-V to LLVM dialect conversion.
I created SPIR-V to LLVM pass, registered it in Passes.td, InitAllPasses.h.
Added a pattern for spv.BitwiseAndOp and tests for it. Integer, float
and vector types are converted through LLVMTypeConverter.
Differential Revision: https://reviews.llvm.org/D81100