[SCEVExpander] Fix a failed cast<> assertion

SCEVExpander::replaceCongruentIVs assumes the backedge value of an
SCEV-analysable PHI to always be an instruction, when this is not
necessarily true.  For now address this by bailing out of the
optimization if the backedge value of the PHI is a non-Instruction.

llvm-svn: 269213