LoongArch: Fix immediate overflow check bug

For B16/B21/B26/PCREL20_S2 relocations, if immediate overflow check after
rightshift, and the mask need to include sign bit.

Now, the immediate overflow check before rightshift for easier understand.

bfd/ChangeLog:

	* elfxx-loongarch.c (reloc_bits_pcrel20_s2): Delete.
	(reloc_bits_b16): Delete.
	(reloc_bits_b21): Delete.
	(reloc_bits_b26): Delete.
	(reloc_sign_bits): New.