preseve SIMD element type information

[folkertdev]

Preserve the SIMD element type and provide it to LLVM for better optimization.

This is relevant for AArch64 types like int16x4x2_t, see also llvm/llvm-project#181514. Such types are defined like so:

#[repr(simd)]
struct int16x4_t([i16; 4]);

#[repr(C)]
struct int16x4x2_t(pub int16x4_t, pub int16x4_t);

Previously this would be translated to the opaque [2 x <8 x i8>], with this PR it is instead [2 x <4 x i16>]. That change is not relevant for the ABI, but using the correct type prevents bitcasts that can (indeed, do) confuse the LLVM pattern matcher.

This change will make it possible to implement the deinterleaving loads on AArch64 in a portable way (without neon-specific intrinsics), which means that e.g. Miri or the cranelift backend can run them without additional support.