[SelectionDAG] Enable CTPOP optimization fine tuning

Add a TLI hook to allow SelectionDAG to fine tune the conversion of CTPOP to a chain of "x & (x - 1)" when CTPOP isn't legal.

A subsequent patch will attempt to fine tune the X86 code gen.

Reviewed By: spatel

Differential Revision: https://reviews.llvm.org/D89952