[SCEV] Check the cache in get{S|U}MaxExpr before doing any work

Summary:
This lets us avoid e.g. checking if A >=s B in getSMaxExpr(A, B) if we've
already established that (A smax B) is the best we can do.

Fixes PR41225.

Reviewers: asbirlea

Subscribers: mcrosier, jlebar, bixia, jdoerfert, llvm-commits

Tags: #llvm

Differential Revision: https://reviews.llvm.org/D60010

llvm-svn: 357320