[SelectionDAG] Initial support for FSHL/FSHR funnel shift opcodes (PR39467)

This is an initial patch to add a minimum level of support for funnel shifts to the SelectionDAG and to begin wiring it up to the X86 SHLD/SHRD instructions.

Some partial legalization code has been added to handle the case for 'SlowSHLD' where we want to expand instead and I've added a few DAG combines so we don't get regressions from the existing DAG builder expansion code.

Differential Revision: https://reviews.llvm.org/D54698

llvm-svn: 348353