[TargetLowering] check for sign-bit comparisons in SimplifyDemandedBits

I don't know if anything other than x86 vectors is affected by this change, but this may allow 
us to remove target-specific intrinsics for blendv* (vector selects). The simplification arises
from the fact that blendv* instructions only use the sign-bit when deciding which vector element
to choose for the destination vector. The mechanism to fold VSELECT into SHRUNKBLEND nodes already
exists in x86 lowering; this demanded bits change just enables the transform to fire more often.

The original motivation starts with a bug for DSE of masked stores that seems completely unrelated, 
but I've explained the likely steps in this series here:
https://llvm.org/bugs/show_bug.cgi?id=11210

Differential Revision: https://reviews.llvm.org/D29687

llvm-svn: 294863