Extend the ValuesAtScope cache to cover all expressions, not just

SCEVUnknowns, as the non-SCEVUnknown cases in the getSCEVAtScope code
can also end up repeatedly climing through the same expression trees,
which can be unusably slow when the trees are very tall.

Also, add a quick check for SCEV pointer equality to the main
SCEV comparison routine, as the full comparison code can be expensive
in the case of large expression trees.

These fix compile-time problems in some pathlogical cases.

llvm-svn: 80623