[LoopFusion] sink second loop PHIs

Fixes https://github.com/llvm/llvm-project/issues/59023

PHI nodes that are in the second loop only have the first loop as its
predecessor. These PHI nodes should be sunk to the end of the fused
loop. If the second loop uses the PHI, then the loops cannot be fused.

I don't think this should happen in typical compilation workflows.
The PHI will be in a dedicated exit block of the first loop following
LCSSA transformations.

Reviewed By: aeubanks

Differential Revision: https://reviews.llvm.org/D139812