[LSR] Fix bug - check if loop has preheader before calling isInductionPHI

Fix bug exposed by https://reviews.llvm.org/D125990
rewriteLoopExitValues calls InductionDescriptor::isInductionPHI which requires
the PHI node to have an incoming edge from the loop preheader. This adds checks
before calling InductionDescriptor::isInductionPHI to see that the loop has a
preheader. Also did some refactoring.

Differential Revision: https://reviews.llvm.org/D129297