LoongArch: Fix symbol size after relaxation

LoongArch: Fix symbol size after relaxation

There's a logic error in loongarch_relax_perform_deletes: when there's
not any delete operation of which the start address is strictly smaller
than the symbol address, splay_tree_predecessor() will return nullptr
and the symbol size will be unchanged even if some bytes of it are
removed.

Make the logic more complete to fix this issue.  Also factor out the
symbol size adjustment logic into a function to avoid code bloating.

Tested-by: WANG Xuerui <git@xen0n.name>
Signed-off-by: Xi Ruoyao <xry111@xry111.site>