[SelectionDAG] ComputeNumSignBits - add support for SMIN+SMAX clamp patterns

If we have a clamp pattern, SMIN(SMAX(X, LO),HI) or SMAX(SMIN(X, HI),LO) then we can deduce that the number of signbits will be at least the minimum of the LO and HI constants.

I haven't bothered with the UMIN/UMAX equivalent as (1) we don't have any current use cases and (2) I wonder if we'd be better off immediately falling back for ComputeKnownBits for UMIN/UMAX which already has optimization patterns useful for unsigned cases.

Differential Revision: https://reviews.llvm.org/D43338

llvm-svn: 325450