[APSInt] Use APInt::compare and APInt::compareSigned to implement APSInt::compareValue

APInt just got compare methods that return -1, 0, or 1 instead of just having ult/slt and eq.

This patch uses these methods to implement APSInt::compareValues so that we don't have to call do an equal comparison and then possibly a second less than comparison.

Differential Revision: https://reviews.llvm.org/D32381

llvm-svn: 301053