[Pipeliner] Fix incorrect phi values in the epilog and kernel

The code that generates the loop definition operand for phis
in the epilog and kernel is incorrect in some cases.

In the kernel, when a phi refers to another phi, the code that
updates PhiOp2 needs to include the stage difference between
the two phis.

In the epilog, the check for using the loop definition instead
of the phi definition uses the StageDiffAdj value (the difference
between the phi stage and the loop definition stage), but the
adjustment is not needed to determine if the current stage
contains an iteration with the loop definition.

Differential Revision: https://reviews.llvm.org/D51167

llvm-svn: 340782