[llvm] Reland "RegisterCoalescer: Add implicit-def of super register when coalescing SUBREG_TO_REG" (PR #123632)

[Matt Arsenault via llvm-commits]

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@@ -1443,6 +1448,9 @@ bool RegisterCoalescer::reMaterializeTrivialDef(const CoalescerPair &CP,
               (MO.getSubReg() == 0 && MO.getReg() == DstOperand.getReg())) &&
              "unexpected implicit virtual register def");
       ImplicitOps.push_back(MO);
+      if (MO.isDef() && MO.getReg().isVirtual() &&
+          MRI->shouldTrackSubRegLiveness(DstReg))
+        NewMIImplicitOpsMask |= MRI->getMaxLaneMaskForVReg(MO.getReg());
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arsenm wrote:

Why is this mixing MO.getReg() and DstReg? 

https://github.com/llvm/llvm-project/pull/123632