[llvm] [VPlan] Run narrowInterleaveGroups during general VPlan optimizations. (PR #149706)

[David Sherwood via llvm-commits]

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david-arm wrote:

Can you add a scalable vector version of at least one of these tests please? I tested this file with this PR and ran `opt -p loop-vectorize -mcpu=neoverse-v1` and we generate IR like this for test_add_double_same_const_args_1:

```
  %wide.load = load <vscale x 2 x double>, ptr %9, align 4
  %wide.load1 = load <vscale x 2 x double>, ptr %10, align 4
  %11 = fadd <vscale x 2 x double> %wide.load, splat (double 1.000000e+00)
  %12 = fadd <vscale x 2 x double> %wide.load1, splat (double 1.000000e+00)
...
  store <vscale x 2 x double> %11, ptr %13, align 4
  store <vscale x 2 x double> %12, ptr %14, align 4
```

https://github.com/llvm/llvm-project/pull/149706