[DebugLoc][Reg-Spiller] Should Spill instructions inherit DebugLoc from their def instruction rather than forward-scanning?

Currently, when the register allocator inserts spill instructions, there is no consistent policy for assigning their DebugLoc. In practice, spill code either ends up with an empty DebugLoc, or uses a forward-scanned location via `findDebugLoc()` logic.

**My question/concern is:** the most appropriate DebugLoc for a spill instruction is inheriting the instruction that defines the register being spilled — i.e., the def instruction immediately preceding the spill, which should use `rfindDebugLoc()` to search. However, no backend in LLVM adopt this method.

For example, here is the debug loc setting following the current fasion:
```
.loc 0 39 10 # control_flow_spill_debugloc.cu:39:10
add.s32 r2 = r2, 119, r0;
.loc 0 40 10 # control_flow_spill_debugloc.cu:40:10
st.local [r1 + 0], r2; # 4-byte Folded Spill
setp p0 = r0, r1;
```

However, I think it can be better if `st.local [r1 + 0], r2;` can use the previous instruction's DebugLoc, i.e. `.loc 0 39 10`.

I have investigated on different backend, there is no unified behavior for every backends unfortunately:

mode | backend | note
-- | -- | --
DebugLoc() | X86、AArch64、AVR | setting an empty DebugLoc for them
I != end ? I->getDebugLoc() : DebugLoc() | ARM/Thumb1/Thumb2、SystemZ、Sparc、LoongArch、PowerPC、Mips、BPF、CSKY、MSP430、Lanai、Xtensa、VE、XCore | inherit Insert point's DebugLoc first, if not, setting an empty
MBB.findDebugLoc(I) | AMDGPU、Hexagon、ARC、M68k | skip debug instruction, and inherit

I wonder is there a convention of setting spill's DebugLoc according to LLVM suggestion? Or do we think this is too trivial to concern about? Thanks a lot in advance!

Start by reviewing the register allocator's spill-instruction insertion paths and the existing findDebugLoc() and rfindDebugLoc() logic. Compare the backend behaviors listed in the issue, then determine whether LLVM wants a unified convention for inheriting a spilled register's defining instruction location; done means the convention and affected backend changes are agreed.

Written by the indexing model from the issue text.