[LoongArch] Merge the 12bit constant address into the offset field of the instruction

There are 12bit offset fields in the ld.[b/h/w/d] and st.[b/h/w/d].
When the constant address is less than 12 bits, the address
calculation is incorporated into the offset field of the instruction.

Differential Revision: https://reviews.llvm.org/D143470