[LoongArch] Emit bytepick for picking from concatenation of two values

It seems the ISA manual's pseudo-code description for the
`BYTEPICK.[WD]` instructions is inaccurate; the behavior described here
should be correct though. The instructions' names are misleading too
(they pick full GRLen-wide words instead of bytes; they just index by
bytes) but let's stick to the official names for now.

Reviewed By: SixWeining

Differential Revision: https://reviews.llvm.org/D143880