[MLIR][LLVMDialect] SelectionOp conversion pattern

This patch introduces conversion pattern for `spv.selection` op.
The conversion can only be applied to selection with all blocks being
reachable. Moreover, selection with control attributes "Flatten" and
"DontFlatten" is not supported.
Since the `PatternRewriter` hook for block merging has not been implemented
for `ConversionPatternRewriter`, merge and continue blocks are kept
separately.

Reviewed By: antiagainst, ftynse

Differential Revision: https://reviews.llvm.org/D83860