[llvm] [RISCV] Add lowerINSERT_SUBVECTORAsBUILD_VECTOR. (PR #110069)

[Craig Topper via llvm-commits]

================
@@ -898,19 +898,19 @@ define <4 x i32> @insert_extract_v8i32_v2i32_0(<2 x i32> %v) {
 define <16 x i32> @build_vector_insert(<16 x i32> %0) {
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topperc wrote:

InstCombine already flattens this. Do you have other examples?

```
define <16 x i32> @build_vector_insert(<16 x i32> %0) {
entry:
  %1 = shufflevector <16 x i32> %0, <16 x i32> <i32 poison, i32 poison, i32 poison, i32 poison, i32 poison, i32 poison, i32 poison, i32 poison, i32 0, i32 0, i32 0, i32 0, i32 0, i32 0, i32 0, i32 0>, <16 x i32> <i32 0, i32 1, i32 2, i32 3, i32 4, i32 5, i32 6, i32 7, i32 24, i32 25, i32 26, i32 27, i32 28, i32 29, i32 30, i32 31>
  ret <16 x i32> %1
}
```

https://github.com/llvm/llvm-project/pull/110069