[LoongArch] Fixup value adjustment in applyFixup

A complete implementation of `applyFixup` for D132323.

Makes `LoongArchAsmBackend::shouldForceRelocation` to determine
if the relocation types must be forced.

This patch also adds range and alignment checks for `b*` instructions'
operands, at which point the offset to a label is known.

Differential Revision: https://reviews.llvm.org/D132818