[VectorCombine] add test to check for iterative improvements; NFC

Author: rotateright

llvm/test/Transforms/VectorCombine/X86/insert-binop.ll

@@ -48,6 +48,26 @@ define <2 x i64> @ins1_ins1_xor(i64 %x, i64 %y) {
   ret <2 x i64> %r
 }
 
+define <2 x i64> @ins1_ins1_iterate(i64 %w, i64 %x, i64 %y, i64 %z) {
+; CHECK-LABEL: @ins1_ins1_iterate(
+; CHECK-NEXT:    [[S0_SCALAR:%.*]] = sub i64 [[W:%.*]], [[X:%.*]]
+; CHECK-NEXT:    [[S0:%.*]] = insertelement <2 x i64> undef, i64 [[S0_SCALAR]], i64 1
+; CHECK-NEXT:    [[I2:%.*]] = insertelement <2 x i64> undef, i64 [[Y:%.*]], i32 1
+; CHECK-NEXT:    [[S1:%.*]] = or <2 x i64> [[S0]], [[I2]]
+; CHECK-NEXT:    [[I3:%.*]] = insertelement <2 x i64> undef, i64 [[Z:%.*]], i32 1
+; CHECK-NEXT:    [[S2:%.*]] = shl <2 x i64> [[I3]], [[S1]]
+; CHECK-NEXT:    ret <2 x i64> [[S2]]
+;
+  %i0 = insertelement <2 x i64> undef, i64 %w, i64 1
+  %i1 = insertelement <2 x i64> undef, i64 %x, i32 1
+  %s0 = sub <2 x i64> %i0, %i1
+  %i2 = insertelement <2 x i64> undef, i64 %y, i32 1
+  %s1 = or <2 x i64> %s0, %i2
+  %i3 = insertelement <2 x i64> undef, i64 %z, i32 1
+  %s2 = shl <2 x i64> %i3, %s1
+  ret <2 x i64> %s2
+}
+
 ; The inserts are free, but it's still better to scalarize.
 
 define <2 x double> @ins0_ins0_fadd(double %x, double %y) {