[LICM] Avoid repeating expensive call while promoting loads. NFC

Summary:
We can avoid repeating the check `isGuaranteedToExecute` when it's already called once while checking if the alignment can be widened for the load/store being hoisted.

The function is invariant for the same instruction `UI` in `isGuaranteedToExecute(*UI, DT, CurLoop, SafetyInfo);`

Reviewers: hfinkel, eli.friedman

Subscribers: llvm-commits

Differential Revision: http://reviews.llvm.org/D21672

llvm-svn: 273671