[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[0])` into `gep &p[n], Base - &p[0]`. 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

llvm-svn: 341516