[LoongArch] Add support for tail call optimization

This patch adds tail call support to the LoongArch backend.  When
appropriate, use the `b` or `jr` instruction for tail calls (the
`pcalau12i+jirl` instruction pair when use medium codemodel).

This patch also modifies the inappropriate operand name:
simm26_bl -> simm26_symbol

This has been modeled after RISCV's tail call opt.

Reviewed By: SixWeining

Differential Revision: https://reviews.llvm.org/D137889