[LoongArch] Make use of addu16i.d for adds with suitable immediates

Ideally `addu16i.d` could be paired with `{ld,st}ptr` for faster memory
accesses with 32-bit-aligned offsets (it was designed for this purpose),
but it would require more work and the original use case (GP-relative
accesses) does not exist any more with the current LoongArch psABI.

It could still be used for accelerating additions of certain constants
though, which is what this patch intends to do.

Reviewed By: SixWeining, gonglingqin

Differential Revision: https://reviews.llvm.org/D143710