[llvm] [HEXAGON] [MachinePipeliner] Fix the DAG in case of dependent phis. (PR #135925)

[Ikhlas Ajbar via llvm-commits]

iajbar wrote:

Abinaya, what do you think about this PR description"
"This change corrects the scheduling relationship between dependent PHI nodes. Previously, the implementation treated SU1 as the successor of SU0. In reality, SU0 should depend on SU1, not the other way around.
The incorrect ordering could cause SU0 to be scheduled before SU1, which leads to invalid IR: subsequent instructions may reference values that have not yet been defined."

https://github.com/llvm/llvm-project/pull/135925