[llvm] [RISCV][GISEL] instruction-select for G_SPLAT_VECTOR (PR #111193)

[Craig Topper via llvm-commits]

================
@@ -67,7 +67,428 @@ define <vscale x 64 x i1> @splat_zero_nxv64i1() {
   ret <vscale x 64 x i1> zeroinitializer
 }
 
+define <vscale x 1 x i8> @splat_zero_nxv1i8() {
----------------
topperc wrote:

The splat pattern in IR is
```
insertelt
shufflevector
```

For scalable vectors, the shufflevector becomes extractelt followed by splat_vector so we get

```
insertelt
extractelt
splatvector
```

We need to combine away the extractelt and insertelt.

https://github.com/llvm/llvm-project/pull/111193