Fix assert in X86 backend.

When running combine on an extract_vector_elt, it wants to look through
a bitcast to check if the argument to the bitcast was itself an
extract_vector_elt with particular operands.

However, it called getOperand() on the argument to the bitcast *before*
checking that the opcode was EXTRACT_VECTOR_ELT, assert-failing if there
were zero operands for the actual opcode.

Fix, and add trivial test.

git-svn-id: https://llvm.org/svn/llvm-project/llvm/trunk@249891 91177308-0d34-0410-b5e6-96231b3b80d8

lib/Target/X86/X86ISelLowering.cpp

@@ -22913,15 +22913,15 @@ static SDValue PerformEXTRACT_VECTOR_ELTCombine(SDNode *N, SelectionDAG &DAG,
                          InputVector.getNode()->getOperand(0));
 
     // The mmx is indirect: (i64 extract_elt (v1i64 bitcast (x86mmx ...))).
-    SDValue MMXSrcOp = MMXSrc.getOperand(0);
     if (MMXSrc.getOpcode() == ISD::EXTRACT_VECTOR_ELT && MMXSrc.hasOneUse() &&
-        MMXSrc.getValueType() == MVT::i64 && MMXSrcOp.hasOneUse() &&
-        MMXSrcOp.getOpcode() == ISD::BITCAST &&
-        MMXSrcOp.getValueType() == MVT::v1i64 &&
-        MMXSrcOp.getOperand(0).getValueType() == MVT::x86mmx)
-      return DAG.getNode(X86ISD::MMX_MOVD2W, SDLoc(InputVector),
-                         N->getValueType(0),
-                         MMXSrcOp.getOperand(0));
+        MMXSrc.getValueType() == MVT::i64) {
+      SDValue MMXSrcOp = MMXSrc.getOperand(0);
+      if (MMXSrcOp.hasOneUse() && MMXSrcOp.getOpcode() == ISD::BITCAST &&
+          MMXSrcOp.getValueType() == MVT::v1i64 &&
+          MMXSrcOp.getOperand(0).getValueType() == MVT::x86mmx)
+        return DAG.getNode(X86ISD::MMX_MOVD2W, SDLoc(InputVector),
+                           N->getValueType(0), MMXSrcOp.getOperand(0));
+    }
   }
 
   EVT VT = N->getValueType(0);

test/CodeGen/Generic/vector.ll

@@ -156,3 +156,9 @@ define <2 x i32*> @vector_gep(<2 x [3 x {i32, i32}]*> %a) {
     %w = getelementptr [3 x {i32, i32}], <2 x [3 x {i32, i32}]*> %a, <2 x i32> <i32 1, i32 2>, <2 x i32> <i32 2, i32 3>, <2 x i32> <i32 1, i32 1>
       ret <2 x i32*> %w
 }
+
+define i32 @extractelt_constant_bitcast() {
+  %1 = bitcast i64 4 to <2 x i32>
+  %2 = extractelement <2 x i32> %1, i32 0
+  ret i32 %2
+}