[MLIR][SPIRVToLLVM] Conversion pattern for loop op

This patch introduces a conversion of `spv.loop` to LLVM dialect.
Similarly to `spv.selection`, op's control attributes are not mapped
to LLVM yet and therefore the conversion fails if the loop control is
not `None`. Also, all blocks within the loop should be reachable in
order for conversion to succeed.

Reviewed By: mravishankar

Differential Revision: https://reviews.llvm.org/D84245