[Polly][DebugInfo] Use getStableDebugLoc to avoid intrinsic-dependent behaviour (#81246)

Polly currently uses `getDebugLoc` in a few places to produce diagnostic
output; this is correct when interacting with specific instructions, but
may be incorrect when dealing with instruction ranges if debug
intrinsics are included. As a general rule, the debug locations attached
to debug intrinsics may be misleading compared to the surrounding
instructions, and are not generally used for anything other than
determining variable scope info; the recommended approach is therefore
to use `getStableDebugLoc` instead, which skips over debug intrinsics.
This is necessary to fix test failures that occur when enabling
non-instruction debug info, which removes debug intrinsics from basic
blocks and thus alters the diagnostic output of Polly (despite causing
no functional change).