RenameIndependentSubregs: Add missing sub-range for new IMPLICIT_DEFs (#89050)

Existing sub-ranges are correctly updated because new IMPLICIT_DEF is
added, but there is missing sub-range for IMPLICIT_DEF itself.
Because of missing sub-range in live-intervals for IMPLICIT_DEF,
register allocator does not know that IMPLICIT_DEF rewrites its
virtual sub-registers and can end up assigning overlapping physical
registers to them.
This results in deleting instructions that were defined by sub-registers
overwritten by IMPLICIT_DEF as they are now dead.

Author: petar-avramovic

llvm/lib/CodeGen/RenameIndependentSubregs.cpp

@@ -334,10 +334,17 @@ void RenameIndependentSubregs::computeMainRangesFixFlags(
                                                DebugLoc(), MCDesc, Reg);
           SlotIndex DefIdx = LIS->InsertMachineInstrInMaps(*ImpDef);
           SlotIndex RegDefIdx = DefIdx.getRegSlot();
+          LaneBitmask Mask = MRI->getMaxLaneMaskForVReg(Reg);
           for (LiveInterval::SubRange &SR : LI.subranges()) {
+            Mask = Mask & ~SR.LaneMask;
             VNInfo *SRVNI = SR.getNextValue(RegDefIdx, Allocator);
             SR.addSegment(LiveRange::Segment(RegDefIdx, PredEnd, SRVNI));
           }
+
+          if (!Mask.none()) {
+            LiveInterval::SubRange *SR = LI.createSubRange(Allocator, Mask);
+            SR->createDeadDef(RegDefIdx, Allocator);
+          }
         }
       }
     }