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

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.

llvm-svn: 186465