[InstCombine] Support disabling expensive combines in opt

Currently, there is no way to disable ExpensiveCombines when doing
a standalone opt -instcombine run, as that's the default, and the
opt option can currently only be used to force enable, not to force
disable. The only way to disable expensive combines is via -O1 or -O2,
but that of course also runs the rest of the kitchen sink...

This patch allows using opt -instcombine -expensive-combines=0 to
run InstCombine without ExpensiveCombines.

Differential Revision: https://reviews.llvm.org/D72861