[IndVars] Turn isValidRewrite into an assertion
Function rewriteLoopExitValues contains a check on isValidRewrite which
is needed to make sure that SCEV does not convert the pattern
gep Base, (&p[n] - &p) into gep &p[n], Base - &p. This problem
has been fixed in SCEV long ago, so this check is just obsolete.
This patch converts it into an assertion to make sure that the SCEV will
not mess up this case in the future.
Differential Revision: https://reviews.llvm.org/D51582
Reviewed By: atrick