[TargetLowering][X86] Teach SimplifyDemandedBits to use ShrinkDemandedOp on ISD::SHL nodes.

If the upper bits of the SHL result aren't used, we might be able to use a narrower shift. For example, on X86 this can turn a 64-bit into 32-bit enabling a smaller encoding.

Differential Revision: https://reviews.llvm.org/D60358

llvm-svn: 358257