[LoongArch] Eliminate extra un-accounted-for successors

Specifically:
```
*** Bad machine code: MBB has unexpected successors which are not branch targets, fallthrough, EHPads, or inlineasm_br targets. ***
- function:    atomicrmw_umax_i8_acquire
- basic block: %bb.3  (0x1b90bd8)

*** Bad machine code: Non-terminator instruction after the first terminator ***
- function:    atomicrmw_umax_i8_acquire
- basic block: %bb.3  (0x1b90bd8)
- instruction: DBAR 1792
```

Differential Revision: https://reviews.llvm.org/D137884