[RegisterCoalescer] Fix subrange update when rematerialization widens a def (#151974)

Currently, when an instruction rematerialized by the register coalescer
defines more subregs of the destination register
than the original COPY instruction did, we only add dead defs for the
newly defined subregs if they were not defined anywhere
else. For example, consider something like this before
rematerialization:
```
 %0:reg64 = CONSTANT 1
 %1:reg128.sub_lo64_lo32 = COPY %0.lo32
 %1:reg128.sub_lo64_hi32 = ...
 ...
```
that would look like this after rematerializing `%0`:
```
 %0:reg64 = CONSTANT 2
 %1:reg128.sub_lo64 = CONSTANT 2
 %1:reg128.sub_lo64_hi32 = ...
 ...
```
A dead def would not be added for `%1.sub_lo64_hi32` at the 2nd
instruction because it's subrange wasn't empty beforehand.

Author: KRM7

llvm/lib/CodeGen/RegisterCoalescer.cpp

@@ -1624,11 +1624,11 @@ bool RegisterCoalescer::reMaterializeTrivialDef(const CoalescerPair &CP,
           UpdatedSubRanges = true;
         } else {
           // We know that this lane is defined by this instruction,
-          // but at this point it may be empty because it is not used by
-          // anything. This happens when updateRegDefUses adds the missing
-          // lanes. Assign that lane a dead def so that the interferences
-          // are properly modeled.
-          if (SR.empty())
+          // but at this point it might not be live because it was not defined
+          // by the original instruction. This happens when the
+          // rematerialization widens the defined register. Assign that lane a
+          // dead def so that the interferences are properly modeled.
+          if (!SR.liveAt(DefIndex))
             SR.createDeadDef(DefIndex, Alloc);
         }
       }

llvm/test/CodeGen/SystemZ/regcoal-subranges-update-remat.mir

@@ -43,3 +43,28 @@ body:             |
     %3:gr32bit = COPY killed %1
     Return implicit %3
 ...
+
+---
+name:            test_dead_at_remat_later_defined
+tracksRegLiveness: true
+body:             |
+  bb.0:
+    ; CHECK-LABEL: name: test_dead_at_remat_later_defined
+    ; CHECK: undef [[LHI:%[0-9]+]].subreg_l32:gr128bit = LHI 0
+    ; CHECK-NEXT: [[LHI:%[0-9]+]].subreg_l64:gr128bit = LGHI 2
+    ; CHECK-NEXT: [[LHI1:%[0-9]+]]:gr32bit = LHI 1
+    ; CHECK-NEXT: [[LHI:%[0-9]+]].subreg_lh32:gr128bit = COPY [[LHI1]]
+    ; CHECK-NEXT: [[LGHI:%[0-9]+]]:gr64bit = LGHI 2
+    ; CHECK-NEXT: [[LHI:%[0-9]+]].subreg_h32:gr128bit = COPY [[LGHI]].subreg_l32
+    ; CHECK-NEXT: $r0q = COPY [[LHI]]
+    ; CHECK-NEXT: $r4d = COPY [[LGHI]].subreg_h32
+    %0:gr64bit = LGHI 2
+    %1:gr32bit = LHI 0
+    %2:gr32bit = LHI 1
+    undef %3.subreg_ll32:gr128bit = COPY %0.subreg_l32
+    %3.subreg_lh32:gr128bit = COPY %2
+    %3.subreg_l32:gr128bit = COPY %1
+    %3.subreg_h32:gr128bit = COPY %0.subreg_l32
+    $r0q = COPY %3
+    $r4d = COPY %0.subreg_h32
+...