[MCP] Disable BackwardCopyPropagateBlock for copies with implicit registers. (#137687)

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 2def1c4, 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 2def1c4.
    
Fixes #131478.

Author: topperc

llvm/lib/CodeGen/MachineCopyPropagation.cpp

@@ -1206,7 +1206,7 @@ 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/pr137687.mir

@@ -0,0 +1,21 @@
+# NOTE: Assertions have been autogenerated by utils/update_mir_test_checks.py UTC_ARGS: --version 5
+# RUN: llc %s -mtriple=s390x-unknown-linux-gnu -run-pass=machine-cp -verify-machineinstrs -o - | FileCheck %s
+
+---
+name:            t
+tracksRegLiveness: true
+noPhis:          true
+isSSA:           false
+noVRegs:         true
+hasFakeUses:     false
+tracksDebugUserValues: true
+body:             |
+  bb.0:
+    ; CHECK-LABEL: name: t
+    ; 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
+    Return implicit $r12q
+...