[MachinePipeliner] Fix Phi refers to Phi in same stage in 1st epilogue
This is exposed by functional testing on PowerPC.
In some pipelined loops, Phi refer to phi did not get value defined by
the Phi, hence getting wrong value later.
As the comment mentioned, we should "use the value defined by the Phi,
unless we're generating the firstepilog and the Phi refers to a Phi
in a different stage.", so Phi refering to same stage Phi should use
the value defined by the Phi here.
Reviewers: bcahoon, hfinkel
Reviewed By: hfinkel
Subscribers: MaskRay, wuzish, nemanjai, hiraditya, llvm-commits
Differential Revision: https://reviews.llvm.org/D64035