[ValueTracking] Teach computeNumSignBits to understand min/max clamp patterns...

[ValueTracking] Teach computeNumSignBits to understand min/max clamp patterns with constant/splat values

If we have a min/max pair we can do a better job of counting sign bits if we look at them together. This is similar to what is done in the SelectionDAG version of computeNumSignBits for ISD::SMAX/SMIN.

Differential Revision: https://reviews.llvm.org/D51112

llvm-svn: 340480