[x86] fix usage of stale operands when lowering select

I noticed this problem as part of the ongoing attempt to canonicalize min/max ops in IR.

The debug output shows nodes like this:

t4: i32 = xor t2, Constant:i32<-1>
    t21: i8 = setcc t4, Constant:i32<0>, setlt:ch
  t14: i32 = select t21, t4, Constant:i32<-1>

And because the select is holding onto the t4 (xor) node while EmitTest creates a new 
x86-specific xor node, the lowering results in:

  t4: i32 = xor t2, Constant:i32<-1>
  t25: i32,i32 = X86ISD::XOR t2, Constant:i32<-1>
t28: i32,glue = X86ISD::CMOV Constant:i32<-1>, t4, Constant:i8<15>, t25:1

Differential Revision: https://reviews.llvm.org/D28374

llvm-svn: 291392

llvm/lib/Target/X86/X86ISelLowering.cpp

@@ -16991,9 +16991,16 @@ SDValue X86TargetLowering::LowerSELECT(SDValue Op, SelectionDAG &DAG) const {
     return DAG.getNode(ISD::EXTRACT_SUBVECTOR, DL, VT, newSelect, zeroConst);
   }
 
-  if (Cond.getOpcode() == ISD::SETCC)
-    if (SDValue NewCond = LowerSETCC(Cond, DAG))
+  if (Cond.getOpcode() == ISD::SETCC) {
+    if (SDValue NewCond = LowerSETCC(Cond, DAG)) {
       Cond = NewCond;
+      // If the condition was updated, it's possible that the operands of the
+      // select were also updated (for example, EmitTest has a RAUW). Refresh
+      // the local references to the select operands in case they got stale.
+      Op1 = Op.getOperand(1);
+      Op2 = Op.getOperand(2);
+    }
+  }
 
   // (select (x == 0), -1, y) -> (sign_bit (x - 1)) | y
   // (select (x == 0), y, -1) -> ~(sign_bit (x - 1)) | y

llvm/test/CodeGen/X86/cmov.ll

@@ -157,16 +157,12 @@ define i8 @test7(i1 inreg %c, i8 inreg %a, i8 inreg %b) nounwind {
   ret i8 %d
 }
 
-; FIXME: The 'not' is redundant.
-
 define i32 @smin(i32 %x) {
 ; CHECK-LABEL: smin:
 ; CHECK:       ## BB#0:
-; CHECK-NEXT:    movl %edi, %ecx
-; CHECK-NEXT:    notl %ecx
 ; CHECK-NEXT:    xorl $-1, %edi
 ; CHECK-NEXT:    movl $-1, %eax
-; CHECK-NEXT:    cmovsl %ecx, %eax
+; CHECK-NEXT:    cmovsl %edi, %eax
 ; CHECK-NEXT:    retq
   %not_x = xor i32 %x, -1
   %1 = icmp slt i32 %not_x, -1