[LICM] Reapply r347776 "Make LICM able to hoist phis" with fix

This commit caused a large compile-time slowdown in some cases when NDEBUG is
off due to the dominator tree verification it added. Fix this by only doing
dominator tree and loop info verification when something has been hoisted.

Differential Revision: https://reviews.llvm.org/D52827

llvm-svn: 347889