[Pipeliner] Fix number of phis to generate in the epilog

The pipeliner was generating too many phis in the epilog blocks, which
caused incorrect code generation when rewriting an instruction that uses
the phi.

In this case, there 3 prolog and epilog stages. An existing phi was
scheduled at stage 1. When generating the code for the 2nd epilog an
extra new phi was generated.

To fix this, we need to update the code that calculates the maximum
number of phis that can be generated, which is based upon the current
prolog stage and the stage of the original phi. In this case, when the
prolog stage is 1 and the original phi stage is 1, the maximum number
of phis to generate is 2.

Patch by Brendon Cahoon.

llvm-svn: 328543