Optimize ScalarEvolution's SCEVComplexityCompare predicate: don't go

scrounging through SCEVUnknown contents and SCEVNAryExpr operands;
instead just do a simple deterministic comparison of the precomputed
hash data.

Also, since this is more precise, it eliminates the need for the slow
N^2 duplicate detection code.

llvm-svn: 105540