[SDAG] Don't require LazyBlockFrequencyInfo at optnone

While LazyBlockFrequencyInfo itself is lazy, the dominator tree
and loop info analyses it requires are not. Drop the dependency
on this pass in SelectionDAGIsel at O0.
This makes for a ~0.6% O0 compile-time improvement.

Differential Revision: https://reviews.llvm.org/D80387