[LoongArch] Use tablegen size for getInstSizeInBytes

Correct the pseudo atomic instruction size for branch
relaxation and branch folding passes.

Inspired by D118175, D118009 and D117970.

Depends on D138481

Reviewed By: SixWeining, gonglingqin, xen0n

Differential Revision: https://reviews.llvm.org/D138469