[LoongArch] Implement isZextFree

This returns true for 8-bit and 16-bit loads, allowing ld.bu/ld.hu to be selected and avoiding unnecessary masks.

Signed-off-by: WANG Rui <wangrui@loongson.cn>

Reviewed By: SixWeining, xen0n

Differential Revision: https://reviews.llvm.org/D154819