Fix the runtime error caused by "Use ValueOffsetPair to enhance value reuse during SCEV expansion".

The patch is to fix the bug in PR28705. It was caused by setting wrong return
value for SCEVExpander::findExistingExpansion. The return values of findExistingExpansion
have different meanings when the function is used in different ways so it is easy to make
mistake. The fix creates two new interfaces to replace SCEVExpander::findExistingExpansion,
and specifies where each interface is expected to be used.

Differential Revision: https://reviews.llvm.org/D22942

llvm-svn: 278161