[LoopSimplifyCFG] Teach LoopSimplifyCFG to constant-fold branches and switches

This patch introduces infrastructure and the simplest case for constant-folding
of branch and switch instructions within loop into unconditional branches.
It is useful as a cleanup for such passes as loop unswitching that sometimes
produce such branches.

Only the simplest case supported in this patch: after the folding, no block
should become dead or stop being part of the loop. Support for more
sophisticated cases will go separately in follow-up patches.

Differential Revision: https://reviews.llvm.org/D54021
Reviewed By: anna

llvm-svn: 347183