[SCEV][NFC] Smarter implementation of isAvailableAtLoopEntry

isAvailableAtLoopEntry duplicates logic of `properlyDominates` after checking invariance.
This patch replaces this logic with invocation of this method which is more profitable
because it supports caching.

Differential Revision: https://reviews.llvm.org/D43997


git-svn-id: https://llvm.org/svn/llvm-project/llvm/trunk@327373 91177308-0d34-0410-b5e6-96231b3b80d8
1 file changed