[LICM] Avoid duplicate work during building AliasSetTracker

Currently we re-use cached info from sub loops or traverse them
to populate AliasSetTracker. But after that we traverse all basic blocks
from the current loop. This is redundant work.

All what we need is traversing the all basic blocks from the loop except
those which are used to get the data from the cache.

This should improve compile time only.

Reviewers: mkazantsev, reames, kariddi, anna
Reviewed By: reames
Subscribers: llvm-commits
Differential Revision: https://reviews.llvm.org/D51715

llvm-svn: 341896