[AArch64] Suboptimal `memset` for non-power of two sizes

[Kmeakin]

Split off from #165625

https://godbolt.org/z/n3qq3EYrT

LLVM assembly:

set4:
        mov     w8, #16843009
        and     w9, w1, #0xff
        mul     w8, w9, w8
        str     w8, [x0]
        ret

GCC assembly:

set4:
        dup     v31.16b, w1
        str     s31, [x0]
        ret

[llvmbot]

@llvm/issue-subscribers-backend-aarch64

Author: Karl Meakin (Kmeakin)

Split off from https://github.com//issues/165625

https://godbolt.org/z/n3qq3EYrT

LLVM assembly:

set4:
        mov     w8, #<!-- -->16843009
        and     w9, w1, #<!-- -->0xff
        mul     w8, w9, w8
        str     w8, [x0]
        ret

GCC assembly:

set4:
        dup     v31.16b, w1
        str     s31, [x0]
        ret