[LoongArch] Modify ParserMethod for the simm26_b operand type

Modify the ParserMethod of `simm26_b` operand type to `parseImmediate`.

Before that, for the `simm26_b` operand type, the same ParserMethod
was used as `simm26_bl`. When using the internal assembler to process
the blockaddress with `asm` instruction, the wrong blockaddress symbol
would be generated due to the call to the `getOrCreateSymbol()`
interface.

Differential Revision: https://reviews.llvm.org/D136073