[GlobalISel] track predecessor mapping during switch lowering.

Correctly populating Machine PHIs relies on knowing exactly how the IR level
CFG was lowered to MachineIR. This needs to be tracked by any translation
phases that meddle (currently only SwitchInst handling).

This reapplies r291973 which was reverted because of testing failures. Fixes:

 + Don't return an ArrayRef to a local temporary.
 + Incorporate Kristof's suggested comment improvements.

llvm-svn: 292278