[AArch64] Use INDEX for constant Neon step vectors (#113424)

When compiling for an SVE target we can use INDEX to generate constant
fixed-length step vectors, e.g.:
```
uint32x4_t foo() {
  return (uint32x4_t){0, 1, 2, 3};
}
```
Currently:
```
foo():
        adrp    x8, .LCPI1_0
        ldr     q0, [x8, :lo12:.LCPI1_0]
        ret
```
With INDEX:
```
foo():
        index   z0.s, #0, #1
        ret
```

The logic for this was already in `LowerBUILD_VECTOR`, though it was
hidden under a check for `!Subtarget->isNeonAvailable()`. This patch
refactors this to enable the corresponding code path unconditionally for
constant step vectors (as long as we can use SVE for them).

0 files changed, 0 insertions, 0 deletions