[TargetLowering][X86] TeachSimplifyDemandedBits to handle cases where only the...

[TargetLowering][X86] TeachSimplifyDemandedBits to handle cases where only the sign bit is demanded from a SETCC and can be passed through

If we're doing a compare that only tests the sign bit and only the sign bit is demanded, we can just bypass the node. This removes one of the blend dependencies in our v2i64->v2f32 uint_to_fp codegen on pre-sse4.2 targets.

Differential Revision: https://reviews.llvm.org/D72356