[DAGCombine] Fix multi-use miscompile in load combine (#81586)

The load combine replaces a number of original loads with one new loads
and also replaces the output chains of the original loads with the
output chain of the new load. This is incorrect if the original load is
retained (due to multi-use), as it may get incorrectly reordered.

Fix this by using makeEquivalentMemoryOrdering() instead, which will
create a TokenFactor with both chains.

Fixes #80911.

Author: nikic

llvm/lib/CodeGen/SelectionDAG/DAGCombiner.cpp

@@ -9252,7 +9252,7 @@ SDValue DAGCombiner::MatchLoadCombine(SDNode *N) {
 
   // Transfer chain users from old loads to the new load.
   for (LoadSDNode *L : Loads)
-    DAG.ReplaceAllUsesOfValueWith(SDValue(L, 1), SDValue(NewLoad.getNode(), 1));
+    DAG.makeEquivalentMemoryOrdering(L, NewLoad);
 
   if (!NeedsBswap)
     return NewLoad;

llvm/test/CodeGen/X86/load-combine.ll

@@ -1283,7 +1283,6 @@ define i32 @zext_load_i32_by_i8_bswap_shl_16(ptr %arg) {
   ret i32 %tmp8
 }
 
-; FIXME: This is a miscompile.
 define i32 @pr80911_vector_load_multiuse(ptr %ptr, ptr %clobber) nounwind {
 ; CHECK-LABEL: pr80911_vector_load_multiuse:
 ; CHECK:       # %bb.0:
@@ -1299,9 +1298,9 @@ define i32 @pr80911_vector_load_multiuse(ptr %ptr, ptr %clobber) nounwind {
 ;
 ; CHECK64-LABEL: pr80911_vector_load_multiuse:
 ; CHECK64:       # %bb.0:
+; CHECK64-NEXT:    movl (%rdi), %ecx
 ; CHECK64-NEXT:    movzwl (%rdi), %eax
 ; CHECK64-NEXT:    movl $0, (%rsi)
-; CHECK64-NEXT:    movl (%rdi), %ecx
 ; CHECK64-NEXT:    movl %ecx, (%rdi)
 ; CHECK64-NEXT:    retq
   %load = load <4 x i8>, ptr %ptr, align 16