[LVI] Cut short search if we know we can't return a useful result

Previously we were recursing on our operands for unary and binary operators regardless of whether we knew how to reason about the operator in question.  This has the effect of doing a potentially large amount of work, only to throw it away.  By checking whether the operation is one LVI can handle, we can cut short the search and return the (overdefined) answer more quickly.  The quality of the results produced should not change.

llvm-svn: 267626