SLPVectorizer: Improve the compile time of isConsecutive by adding a simple constant...

SLPVectorizer: Improve the compile time of isConsecutive by adding a simple constant-gep check before using SCEV.
This check does not always work because not all of the GEPs use a constant offset, but it happens often enough to reduce the number of times we use SCEV.

git-svn-id: https://llvm.org/svn/llvm-project/llvm/trunk@186465 91177308-0d34-0410-b5e6-96231b3b80d8