[SimplifyCFG] use profile metadata to refine merging branch conditions

This is one step towards solving:
https://llvm.org/PR49336

In that example, we disregard the recommended usage of builtin_expect,
so an expensive (unpredictable) branch is folded into another branch
that is guarding it.
Here, we read the profile metadata to see if the 1st (predecessor)
condition is likely to cause execution to bypass the 2nd (successor)
condition before merging conditions by using logic ops.

Differential Revision: https://reviews.llvm.org/D98898