[MLIR][SPIRVToLLVM] Conversion of SPIR-V branch ops

This patch introduces conversion for `spv.Branch` and `spv.BranchConditional`
ops. Branch weigths for `spv.BranchConditional` are not supported at the
moment, and conversion in this case fails.

Reviewed By: antiagainst

Differential Revision: https://reviews.llvm.org/D83784