[InstCombine] Pull shuffles out of binops with splatted ops (#137948)

Given a binary op on splatted vector and a splatted constant,
InstCombine will normally pull the shuffle out in
`InstCombinerImpl::foldVectorBinop`:

```llvm
define <4 x i32> @f(i32 %x) {
  %x.insert = insertelement <4 x i32> poison, i32 %x, i64 0
  %x.splat = shufflevector <4 x i32> %x.insert, <4 x i32> poison, <4 x i32> zeroinitializer
  %res = add <4 x i32> %x.splat, splat (i32 42)
  ret <4 x i32> %res
}
```

```llvm
define <4 x i32> @f(i32 %x) {
  %x.insert = insertelement <4 x i32> poison, i32 %x, i64 0
  %1 = add <4 x i32> %x.insert, <i32 42, i32 poison, i32 poison, i32 poison>
  %res = shufflevector <4 x i32> %1, <4 x i32> poison, <4 x i32> zeroinitializer
  ret <4 x i32> %res
}
```

However, this currently only operates on fixed length vectors. Splats of
scalable vectors don't currently have their shuffle pulled out, e.g:

```llvm
define <vscale x 4 x i32> @f(i32 %x) {
  %x.insert = insertelement <vscale x 4 x i32> poison, i32 %x, i64 0
  %x.splat = shufflevector <vscale x 4 x i32> %x.insert, <vscale x 4 x i32> poison, <vscale x 4 x i32> zeroinitializer
  %res = add <vscale x 4 x i32> %x.splat, splat (i32 42)
  ret <vscale x 4 x i32> %res
}
```

Having this canonical form with the shuffle pulled out is important as
VectorCombine relies on it in order to scalarize binary ops in
`scalarizeBinopOrCmp`, which would prevent the need for #137786. This
also brings it in line for scalable binary ops with two non-constant
operands: https://godbolt.org/z/M9f7ebzca

This adds a combine just after the fixed-length version, but restricted
to splats at index 0 so that it also handles the scalable case:

So the whilst the existing combine looks like: `Op(shuffle(V1, Mask), C)
-> shuffle(Op(V1, NewC), Mask)`

This patch adds: `Op(shuffle(V1, 0), (splat C)) -> shuffle(Op(V1, (splat
C)), 0)`

I think this could be generalized to other splat indexes that aren't
zero, but I think it would be dead code since only fixed-length vectors
can have non-zero shuffle indices, which would be covered by the
existing combine.

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