[SelectionDAGBuilder] Enable funnel shift building to custom rotates

This patch enables funnel shift -> rotate building for all ROTL/ROTR custom/legal operations.

AFAICT X86 was the last target that was missing modulo support (PR38243), but I've tried to CC stakeholders for every target that has ROTL/ROTR custom handling for their final OK.

Differential Revision: https://reviews.llvm.org/D55747

llvm-svn: 349765