[LoongArch] Add emergency spill slot for CFR spill/reload

When all registers have been allocated and CFR needs to be saved on the
stack, an emergency spill slot is required. Because CFR's spill and
reload require a general purpose register to transfer.

The attached test case was bugpoint-reduced down from
`MultiSource/Benchmarks/mafft/Lalignmm.c` in the test-suite.
Without this patch, llc will crash and report the following errors:

```
LLVM ERROR: Error while trying to spill R4 from class GPR: Cannot scavenge register without an emergency spill slot!
```

Reviewed By: SixWeining

Differential Revision: https://reviews.llvm.org/D138007