[RegisterCoalescer] Mark implicit-defs of super-registers as dead in remat

Currently, something like:

```
$eax = MOV32ri -11, implicit-def $rax
%al = COPY $eax
```

Can be rematerialized as:
```
dead $eax = MOV32ri -11, implicit-def $rax
```

Which marks the full $rax as used, not just $al.

With this change, this is rematerialized as:

```
dead $eax = MOV32ri -11, implicit-def dead $rax, implicit-def $al
```

To indicate that only $al is used. This issue is latent right now, but
is exposed when llvm#134408 is applied, as it results in the register
pressure being incorrectly calculated.

I think this change is in line with past fixes in this area, notably:
llvm@059cead
llvm@69cd121

Author: MacDue

llvm/lib/CodeGen/RegisterCoalescer.cpp

@@ -1475,7 +1475,8 @@ bool RegisterCoalescer::reMaterializeTrivialDef(const CoalescerPair &CP,
   // The implicit-def of the super register may have been reduced to
   // subregisters depending on the uses.
 
-  bool NewMIDefinesFullReg = false;
+  TinyPtrVector<MachineOperand *> NewMIImpDefDestReg;
+  [[maybe_unused]] unsigned NewMIOpCount = NewMI.getNumOperands();
 
   SmallVector<MCRegister, 4> NewMIImplDefs;
   for (unsigned i = NewMI.getDesc().getNumOperands(),
@@ -1486,7 +1487,7 @@ bool RegisterCoalescer::reMaterializeTrivialDef(const CoalescerPair &CP,
       assert(MO.isImplicit());
       if (MO.getReg().isPhysical()) {
         if (MO.getReg() == DstReg)
-          NewMIDefinesFullReg = true;
+          NewMIImpDefDestReg.push_back(&MO);
 
         assert(MO.isImplicit() && MO.getReg().isPhysical() &&
                (MO.isDead() ||
@@ -1640,9 +1641,32 @@ bool RegisterCoalescer::reMaterializeTrivialDef(const CoalescerPair &CP,
     // been asked for. If so it must implicitly define the whole thing.
     assert(DstReg.isPhysical() &&
            "Only expect virtual or physical registers in remat");
+
+    // When we're rematerializing into a not-quite-right register we already add
+    // the real definition as an implicit-def, but we should also be marking the
+    // "official" register as dead, since nothing else is going to use it as a
+    // result of this remat. Not doing this can affect pressure tracking.
     NewMI.getOperand(0).setIsDead(true);
 
-    if (!NewMIDefinesFullReg) {
+    bool HasDefMatchingCopy = false;
+    if (!NewMIImpDefDestReg.empty()) {
+      // Assert to check MachineOperand*s have not been invalidated.
+      assert(
+          NewMIOpCount == NewMI.getNumOperands() &&
+          "Expected NewMI operands not to be appended/removed at this point");
+      // If NewMI has an implicit-def of a super-register of the CopyDstReg,
+      // we must also mark that as dead since it is not going to used as a
+      // result of this remat.
+      for (MachineOperand *MO : NewMIImpDefDestReg) {
+        if (MO->getReg() != CopyDstReg)
+          MO->setIsDead(true);
+        else
+          HasDefMatchingCopy = true;
+      }
+    }
+
+    // If NewMI does not already have an implicit-def CopyDstReg add one now.
+    if (!HasDefMatchingCopy) {
       NewMI.addOperand(MachineOperand::CreateReg(
           CopyDstReg, true /*IsDef*/, true /*IsImp*/, false /*IsKill*/));
     }

llvm/test/CodeGen/X86/rematerialize-sub-super-reg.mir

@@ -167,16 +167,14 @@ body:             |
     RET 0, $rax
 ...
 ---
-; FIXME: `$al = COPY killed %4` should rematerialize as `dead $eax = MOV32r0 ... implicit-def $al`
-;         not `dead $eax = MOV32r0 ... implicit-def $rax` (as the full $rax is not used).
 name:  rematerialize_superregister_into_subregister_def_with_impdef_physreg
 body:             |
   bb.0.entry:
     ; CHECK-LABEL: name: rematerialize_superregister_into_subregister_def_with_impdef_physreg
     ; CHECK: dead $esi = MOV32r0 implicit-def dead $eflags, implicit-def $rsi
     ; CHECK-NEXT: dead $edx = MOV32r0 implicit-def dead $eflags, implicit-def $rdx
     ; CHECK-NEXT: FAKE_USE implicit killed $rsi, implicit killed $rdx
-    ; CHECK-NEXT: dead $eax = MOV32r0 implicit-def dead $eflags, implicit-def $rax
+    ; CHECK-NEXT: dead $eax = MOV32r0 implicit-def dead $eflags, implicit-def dead $rax, implicit-def $al
     ; CHECK-NEXT: FAKE_USE implicit killed $al
     ; CHECK-NEXT: $eax = MOV32r0 implicit-def dead $eflags
     ; CHECK-NEXT: RET 0, $eax