[GlobalISel] Enable usage of BranchProbabilityInfo in IRTranslator.

[GlobalISel] Enable usage of BranchProbabilityInfo in IRTranslator.

We weren't using this before, so none of the MachineFunction CFG edges had the
branch probability information added. As a result, block placement later in the
pipeline was flying blind.

This is enabled only with optimizations enabled like SelectionDAG.

Differential Revision: https://reviews.llvm.org/D86824