[llvm] [VectorCombine] Combine scalar fneg with insert/extract to vector fneg when length is different (PR #115209)

[Simon Pilgrim via llvm-commits]

================
@@ -46,6 +72,25 @@ define <2 x double> @ext1_v2f64(<2 x double> %x, <2 x double> %y) {
   ret <2 x double> %r
 }
 
+define <4 x double> @ext1_v2f64v4f64(<2 x double> %x, <4 x double> %y) {
+; SSE-LABEL: @ext1_v2f64v4f64(
+; SSE-NEXT:    [[E:%.*]] = extractelement <2 x double> [[X:%.*]], i32 1
+; SSE-NEXT:    [[N:%.*]] = fneg nsz double [[E]]
+; SSE-NEXT:    [[R:%.*]] = insertelement <4 x double> [[Y:%.*]], double [[N]], i32 1
+; SSE-NEXT:    ret <4 x double> [[R]]
+;
+; AVX-LABEL: @ext1_v2f64v4f64(
+; AVX-NEXT:    [[TMP1:%.*]] = fneg nsz <2 x double> [[X:%.*]]
+; AVX-NEXT:    [[TMP2:%.*]] = shufflevector <2 x double> [[X]], <2 x double> poison, <4 x i32> <i32 poison, i32 1, i32 poison, i32 poison>
+; AVX-NEXT:    [[R:%.*]] = shufflevector <4 x double> [[Y:%.*]], <4 x double> [[TMP2]], <4 x i32> <i32 0, i32 5, i32 2, i32 3>
+; AVX-NEXT:    ret <4 x double> [[R]]
+;
+  %e = extractelement <2 x double> %x, i32 1
+  %n = fneg nsz double %e
+  %r = insertelement <4 x double> %y, double %n, i32 1
+  ret <4 x double> %r
+}
----------------
RKSimon wrote:

Please can you add coverage from when you extract from an index greater than the vector width of the destination:
```
  %e = extractelement <4 x double> %x, i32 3
  %n = fneg nsz double %e
  %r = insertelement <2 x double> %y, double %n, i32 1
  ret <4 x double> %r
```

https://github.com/llvm/llvm-project/pull/115209