[llvm] [PHIElimination] Reuse existing COPY in predecessor basic block (PR #131837)

[Mikael Holmén via llvm-commits]

================
@@ -581,6 +581,15 @@ void PHIEliminationImpl::LowerPHINode(MachineBasicBlock &MBB,
       continue;
     }
 
+    // Reuse an existing copy in the block if possible.
+    if (MachineInstr *DefMI = MRI->getUniqueVRegDef(SrcReg)) {
+      if (DefMI->isCopy() && DefMI->getParent() == &opBlock &&
+          MRI->use_empty(SrcReg)) {
----------------
mikaelholmen wrote:

I also wonder if something more is needed to make debug-info as good as possible in case this optimization triggers.
I tried changing to use_nodbg_empty locally, and then I noticed that
```test/DebugInfo/ARM/partial-subreg.ll``` failed.
After PHI elimination we then got
```
  %18:dpair = COPY killed %11:qpr
  DBG_VALUE %5:dpair, $noreg, !"self", !DIExpression(DW_OP_LLVM_fragment, 0, 96), debug-location !9; simd.swift:5897:5 line no:5897
```
instead of the previous
```
  %5:dpair = COPY killed %11:qpr
  DBG_VALUE %5:dpair, $noreg, !"self", !DIExpression(DW_OP_LLVM_fragment, 0, 96), debug-location !9; simd.swift:5897:5 line no:5897
```
i.e. tthe DBG_VALUE was leftt witth a reference to %5 which gott no definition.

https://github.com/llvm/llvm-project/pull/131837