[Reassociate] add tests for vector negate with undef elements; NFC

Reduced/expanded from issue llvm#57683.

Author: rotateright

llvm/test/Transforms/Reassociate/negation.ll

@@ -1,5 +1,5 @@
 ; NOTE: Assertions have been autogenerated by utils/update_test_checks.py
-; RUN: opt < %s -reassociate -instcombine -S | FileCheck %s
+; RUN: opt < %s -reassociate -S | FileCheck %s
 
 ; Test that we can turn things like X*-(Y*Z) -> X*-1*Y*Z.
 
@@ -44,3 +44,34 @@ define <2 x i32> @negate_vec_undefs(<2 x i32> %a, <2 x i32> %b, <2 x i32> %z) {
   ret <2 x i32> %f
 }
 
+; FIXME: Replacing x with a partial undef negation is a miscompile.
+
+define <2 x i32> @PR57683(<2 x i32> %x) {
+; CHECK-LABEL: @PR57683(
+; CHECK-NEXT:    [[PARTIAL_NEG:%.*]] = sub <2 x i32> <i32 poison, i32 0>, [[X:%.*]]
+; CHECK-NEXT:    [[SHUF:%.*]] = shufflevector <2 x i32> [[PARTIAL_NEG]], <2 x i32> [[X]], <2 x i32> <i32 1, i32 3>
+; CHECK-NEXT:    [[SUB:%.*]] = add <2 x i32> [[PARTIAL_NEG]], <i32 1, i32 1>
+; CHECK-NEXT:    [[R:%.*]] = add <2 x i32> [[SUB]], [[SHUF]]
+; CHECK-NEXT:    ret <2 x i32> [[R]]
+;
+  %partial_neg = sub <2 x i32> <i32 poison, i32 0>, %x
+  %shuf = shufflevector <2 x i32> %partial_neg, <2 x i32> %x, <2 x i32> <i32 1, i32 3>
+  %sub = sub <2 x i32> <i32 1, i32 1>, %x
+  %r = add <2 x i32> %sub, %shuf
+  ret <2 x i32> %r
+}
+
+define <2 x float> @PR57683_FP(<2 x float> %x) {
+; CHECK-LABEL: @PR57683_FP(
+; CHECK-NEXT:    [[PARTIAL_NEG:%.*]] = fsub reassoc nsz <2 x float> <float poison, float 0.000000e+00>, [[X:%.*]]
+; CHECK-NEXT:    [[SHUF:%.*]] = shufflevector <2 x float> [[PARTIAL_NEG]], <2 x float> [[X]], <2 x i32> <i32 1, i32 3>
+; CHECK-NEXT:    [[SUB:%.*]] = fadd reassoc nsz <2 x float> [[PARTIAL_NEG]], <float 1.000000e+00, float 1.000000e+00>
+; CHECK-NEXT:    [[R:%.*]] = fadd reassoc nsz <2 x float> [[SUB]], [[SHUF]]
+; CHECK-NEXT:    ret <2 x float> [[R]]
+;
+  %partial_neg = fsub reassoc nsz <2 x float> <float poison, float 0.0>, %x
+  %shuf = shufflevector <2 x float> %partial_neg, <2 x float> %x, <2 x i32> <i32 1, i32 3>
+  %sub = fsub reassoc nsz <2 x float> <float 1.0, float 1.0>, %x
+  %r = fadd reassoc nsz <2 x float> %sub, %shuf
+  ret <2 x float> %r
+}