[llvm] [SLP]Initial non-power-of-2 (but still whole register) for remaining nodes (PR #113356)

[Sushant Gokhale via llvm-commits]

sushgokh wrote:

@alexey-bataev you might also want to consider this example:

```
define <6 x double> @load_6xdouble(ptr %a)
{
entry:
        %1 = load double, ptr %a, align 8
        %2 = getelementptr double, ptr %a, i16 1
        %3 = load double, ptr %2, align 8
        %4 = getelementptr double, ptr %a, i16 2
        %5 = load double, ptr %4, align 8
        %6 = getelementptr double, ptr %a, i16 3
        %7 = load double, ptr %6, align 8
        %8 = getelementptr double, ptr %a, i16 4
        %9 = load double, ptr %8, align 8
        %10 = getelementptr double, ptr %a, i16 5
        %11 = load double, ptr %10, align 8
        %12 = insertelement <6 x double> poison, double %1, i32 0
        %13 = insertelement <6 x double> %12, double %3, i32 1
        %14 = insertelement <6 x double> %13, double %5, i32 2
        %15 = insertelement <6 x double> %14, double %7, i32 3
        %16 = insertelement <6 x double> %15, double %9, i32 4
        %17 = insertelement <6 x double> %16, double %11, i32 5
        ret <6 x double> %17
}
```

ToT output:
```
define <6 x double> @load_6xdouble(ptr %a) {
entry:
  %0 = load <4 x double>, ptr %a, align 8
  %1 = getelementptr double, ptr %a, i16 4
  %2 = load <2 x double>, ptr %1, align 8
  %3 = shufflevector <4 x double> %0, <4 x double> poison, <6 x i32> <i32 0, i32 1, i32 2, i32 3, i32 poison, i32 poison>
  %4 = shufflevector <2 x double> %2, <2 x double> poison, <6 x i32> <i32 0, i32 1, i32 poison, i32 poison, i32 poison, i32 poison>
  %5 = shufflevector <6 x double> %3, <6 x double> %4, <6 x i32> <i32 0, i32 1, i32 2, i32 3, i32 6, i32 7>
  ret <6 x double> %5
}
```

Expected output:
```
define <6 x double> @load_6xdouble(ptr %a) {
entry:
  %0 = load <6 x double>, ptr %a, align 8
  ret <6 x double> %0
}

I hope I am not missing out on some issue with not having this
```

https://github.com/llvm/llvm-project/pull/113356