[HEXAGON] [MachinePipeliner] Fix the DAG in case of dependent phis. (#135925)
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.
%3:intregs = PHI %21:intregs, %bb.6, %7:intregs, %bb.1 - SU0 %7:intregs
= PHI %21:intregs, %bb.6, %13:intregs, %bb.1 - SU1 %27:intregs = A2_zxtb
%3:intregs - SU2
%13:intregs = C2_muxri %45:predregs, 0, %46:intreg
Co-Authored by: Sumanth Gundapaneni