[SelectionDAG] Add the option of disabling generic combines.

Summary:
For some targets generic combines don't really do much and they
consume a disproportionate amount of time.
There's not really a mechanism in SDISel to tactically disable
combines, but we can have a switch to disable all of them and
let the targets just implement what they specifically need.

Subscribers: hiraditya, llvm-commits

Tags: #llvm

Differential Revision: https://reviews.llvm.org/D79112