[DAG] SimplifyDemandedBits - add sra(shl(x,c1),c1) -> sign_extend_inreg(x) demanded elts fold

Move the sra(shl(x,c1),c1) -> sign_extend_inreg(x) fold inside SimplifyDemandedBits so we can recognize hidden splats with DemandedElts masks.

Because the c1 shift amount has multiple uses, hidden splats won't get simplified to a splat constant buildvector - meaning the existing fold in DAGCombiner::visitSRA can't fire as it won't see a uniform shift amount.

I also needed to add TLI preferSextInRegOfTruncate hook to help keep truncate(sign_extend_inreg(x)) vector patterns on X86 so we can use PACKSS more efficiently.

Differential Revision: https://reviews.llvm.org/D157972