[ConstantRange] Implement getSignedMin/Max in a less complicated and faster way

Summary: As far as I can tell we should be able to implement these almost the same way we do unsigned, but using signed comparisons and checks for min signed value instead of min unsigned value.

Reviewers: pete, davide, sanjoy

Reviewed By: davide

Subscribers: llvm-commits

Differential Revision: https://reviews.llvm.org/D33815

llvm-svn: 305607