[AArch64] Keep floating-point conversion in SIMD (#147707)
Stores can be issued faster if the result is kept in the SIMD/FP
registers.
The `HasOneUse` guards against creating two floating point conversions,
if for example there's some arithmetic done on the converted value as
well. Another approach would be to inspect the user instructions during
lowering, but I don't see that type of check in the lowering too often.
