[LoopDeletion] Handle switch in proving that loop exits on first iteration

Added check for switch-terminated blocks in loops.
Now if a block is terminated with a switch, we try to find out which of the
cases is taken on 1st iteration and mark corresponding edge from the block
to the case successor as live.

Patch by Dmitry Makogon!

Differential Revision: https://reviews.llvm.org/D105688
Reviewed By: nikic, mkazantsev