[LoongArch] Use empty debug location for register spill/reload

Spill/reload instructions are automatically generated by the
compiler and have no relation to the original source code. So it
would be better to not attach any debug location to them.
The X86/AArch64/ARM/Thumb backends all follow this way.

Reviewed By: xen0n

Differential Revision: https://reviews.llvm.org/D148304