[SelectionDAG][X86] CombineBT - more aggressively determine demanded bits

This patch is in 2 parts:

1 - replace combineBT's use of SimplifyDemandedBits (hasOneUse only) with SelectionDAG::GetDemandedBits to more aggressively determine the lower bits used by BT.

2 - update SelectionDAG::GetDemandedBits to support ANY_EXTEND - if the demanded bits are only in the non-extended portion, then peek through and demand from the source value and then ANY_EXTEND that if we found a match.

Differential Revision: https://reviews.llvm.org/D35896

llvm-svn: 309486