[TargetLowering][AMDGPU][X86] Improve SimplifyDemandedBits bitcast handling

This patch adds support for BigBitWidth -> SmallBitWidth bitcasts, splitting the DemandedBits/Elts accordingly.

The AMDGPU backend needed an extra  (srl (and x, c1 << c2), c2) -> (and (srl(x, c2), c1) combine to encourage BFE creation, I investigated putting this in DAGCombine but it caused a lot of noise on other targets - some improvements, some regressions.

The X86 changes are all definite wins.

Differential Revision: https://reviews.llvm.org/D60462

llvm-svn: 358887