[ValueTracking] improve ComputeNumSignBits for vector constants

[ValueTracking] improve ComputeNumSignBits for vector constants

This is similar to the computeKnownBits improvement in rL268479.
There's probably more we can do for vector logic instructions, but
this should let us see non-splat constant masking ops that can
become vector selects instead of and/andn/or sequences.

Differential Revision: http://reviews.llvm.org/D21610

git-svn-id: https://llvm.org/svn/llvm-project/llvm/trunk@273459 91177308-0d34-0410-b5e6-96231b3b80d8