[NVPTX][DebugInfo] avoid emitting extra .loc directives (#84584)

This change removes an extra, unneeded debug directive emitted in the
PTX at the beginning on non-empty functions:
```nvptx
.visible .func  (.param .b32 func_retval0) foo(
        .param .b32 foo_param_0,
        .param .b32 foo_param_1
)
{
        .reg .b32    %r<4>;
        .loc    1 26 0             <---- unneeded (removed by the PR)
$L__func_begin0:
        .loc    1 26 0
```

Author: AlexMaclean

llvm/lib/Target/NVPTX/NVPTXAsmPrinter.cpp

@@ -489,8 +489,11 @@ void NVPTXAsmPrinter::emitFunctionEntryLabel() {
   OutStreamer->emitRawText(StringRef("{\n"));
   setAndEmitFunctionVirtualRegisters(*MF);
   // Emit initial .loc debug directive for correct relocation symbol data.
-  if (MMI && MMI->hasDebugInfo())
-    emitInitialRawDwarfLocDirective(*MF);
+  if (const DISubprogram *SP = MF->getFunction().getSubprogram()) {
+    assert(SP->getUnit());
+    if (!SP->getUnit()->isDebugDirectivesOnly() && MMI && MMI->hasDebugInfo())
+      emitInitialRawDwarfLocDirective(*MF);
+  }
 }
 
 bool NVPTXAsmPrinter::runOnMachineFunction(MachineFunction &F) {

llvm/test/DebugInfo/NVPTX/no-extra-loc.ll

@@ -0,0 +1,26 @@
+; RUN: llc < %s -mtriple=nvptx64-nvidia-cuda | FileCheck %s
+; RUN: %if ptxas %{ llc < %s -mtriple=nvptx64-nvidia-cuda | %ptxas-verify %}
+
+
+define i32 @foo(i32 %a, i32 %b) !dbg !3 {
+
+; CHECK:     .loc    [[FILE:[0-9]+]] 26 0          // extra-lineinfo.cu:26:0
+; CHECK-NOT: .loc    [[FILE]]        26 0          // extra-lineinfo.cu:26:0
+; CHECK:     .file   [[FILE]] "/test/directory/extra-lineinfo.cu"
+
+  %add = add i32 %b, %a, !dbg !6
+  ret i32 %add, !dbg !6
+}
+
+!llvm.dbg.cu = !{!0}
+!nvvm.annotations = !{}
+!llvm.module.flags = !{!2}
+
+!0 = distinct !DICompileUnit(language: DW_LANG_C_plus_plus, file: !1, producer: "clang", isOptimized: true, runtimeVersion: 0, emissionKind: DebugDirectivesOnly)
+!1 = !DIFile(filename: "extra-lineinfo.cu", directory: "/test/directory/")
+!2 = !{i32 1, !"Debug Info Version", i32 3}
+!3 = distinct !DISubprogram(name: "kernel", linkageName: "foo", scope: !1, file: !1, line: 123, type: !4, scopeLine: 26, spFlags: DISPFlagDefinition | DISPFlagOptimized, unit: !0)
+!4 = !DISubroutineType(types: !5)
+!5 = !{}
+!6 = !DILocation(line: 40, column: 22, scope: !31)
+!31 = distinct !DILexicalBlock(scope: !3, file: !1, line: 3, column: 17)