[XRay] Debug instructions count toward xray-instruction-threshold

Debug information changes the generated code when selective XRay
instrumentation is enabled. LLVM counts debug-only `MachineInstr`s toward
`xray-instruction-threshold`, so the same function can be instrumented with
`-g` but not with `-g0`.

This is a code-generation quality bug rather than a miscompile. It reproduces
on x86-64 with LLVM 24.0.0git at llvm-project revision
`2e3d50b4111f324fb615c3c18d02189caa1ef62d`.

Reproducer

```c
__attribute__((noinline)) int xray_frontier(int x) {
return x + 1;
}
```

Compile the same source twice:

```sh
clang -O2 -fxray-instrument -fxray-instruction-threshold=4 \
-g0 -c reduced.c -o g0.o
clang -O2 -fxray-instrument -fxray-instruction-threshold=4 \
-g -c reduced.c -o debug.o

llvm-readobj --sections g0.o | grep -E 'xray_(instr_map|fn_idx)'
llvm-readobj --sections debug.o | grep -E 'xray_(instr_map|fn_idx)'
```

The first query has no output. The second reports both sections:

```text
Name: xray_instr_map
Name: xray_fn_idx
```

The `-g0` object contains only the ordinary `lea` and `ret` instructions for
`xray_frontier`. The `-g` object also contains an XRay entry sled.

## Causal evidence

The Machine IR can be dumped with:

```sh
clang -O2 -fxray-instrument -fxray-instruction-threshold=4 -g0 \
-mllvm -print-before=xray-instrumentation \
-mllvm -filter-print-funcs=xray_frontier -c reduced.c -o /dev/null

clang -O2 -fxray-instrument -fxray-instruction-threshold=4 -g \
-mllvm -print-before=xray-instrumentation \
-mllvm -filter-print-funcs=xray_frontier -c reduced.c -o /dev/null
```

Immediately before `xray-instrumentation`, the `-g0` function has three
MachineInstrs:

```text
KILL
LEA64_32r
RET64
```

The `-g` function has the same three ordinary instructions and one additional
debug instruction:

```text
DBG_VALUE
KILL
LEA64_32r
RET64
```

`XRayInstrumentation.cpp` currently computes the threshold count with:

```cpp
uint64_t MICount = 0;
for (const auto &MBB : MF)
MICount += MBB.size();
```

The raw counts are therefore three and four. At threshold four, the `-g0`
function is below the threshold, while the debug build is not.

## Expected behavior

Debug information should not affect the XRay instrumentation decision. The
threshold should count non-debug machine instructions. This report is limited
to `MachineInstr::isDebugInstr()` records; changing the treatment of pseudo
probes or other non-debug meta instructions is outside its scope.

Start in XRayInstrumentation.cpp at the MICount calculation and compare its handling with MachineInstr::isDebugInstr(). Re-run the supplied clang reproducer and Machine IR dumps with -g0 and -g; done means debug instructions no longer change whether the function crosses xray-instruction-threshold.

Written by the indexing model from the issue text.