[LowerExpectIntrinsic] Propagate branch weights through phi values when...

[LowerExpectIntrinsic] Propagate branch weights through phi values when ExpectedValue is unlikely in LowerExpectIntrinsic

Update handlePhiDef to consider the probability argument in an expect.with.probability intrinsic when annotating BranchInsts.
In addition, we also disallow non-constant probability arguments in this intrinsic.

Differential Revsion: https://reviews.llvm.org/D140337