[MCP] Disable BackwardCopyPropagateBlock for copies with implicit registers.

If there's an implicit-def of a super register, the propagation
must preserve this implicit-def. Knowing how and when to do this
may require target specific knowledge so just disable it for now.

Prior to e17f07c, we checked that the copy had explicit 2 operands
when that was removed we started allowing implicit operands through.
This patch adds a check for implicit operands, but still allows
extra explicit operands which was the goal of e17f07c.

Fixes #137667.

Author: topperc

llvm/lib/CodeGen/MachineCopyPropagation.cpp

@@ -1200,7 +1200,7 @@ void MachineCopyPropagation::BackwardCopyPropagateBlock(
     // Ignore non-trivial COPYs.
     std::optional<DestSourcePair> CopyOperands =
         isCopyInstr(MI, *TII, UseCopyInstr);
-    if (CopyOperands) {
+    if (CopyOperands && MI.getNumImplicitOperands() == 0) {
       Register DefReg = CopyOperands->Destination->getReg();
       Register SrcReg = CopyOperands->Source->getReg();
 

llvm/test/CodeGen/SystemZ/pr137667.mir

@@ -12,7 +12,8 @@ tracksDebugUserValues: true
 body:             |
   bb.0:
     ; CHECK-LABEL: name: t
-    ; CHECK: renamable $r12d = LLILL 0
+    ; CHECK: renamable $r14d = LLILL 0
+    ; CHECK-NEXT: renamable $r12d = COPY killed renamable $r14d, implicit-def $r12q
     ; CHECK-NEXT: Return implicit $r12q
     renamable $r14d = LLILL 0
     renamable $r12d = COPY killed renamable $r14d, implicit-def $r12q