[AMDGPU] Remove dead function metadata after amdgpu-lower-kernel-arguments (llvm#126147)

The verifier ensures function !dbg metadata is unique across the module,
so ensure the old nameless function we leave behind doesn't violate
this invariant.

Removing the function via e.g. eraseFromParent seems like a better
option, but doesn't seem to be legal from a FunctionPass.

Author: slinder1

llvm/lib/Target/AMDGPU/AMDGPULowerKernelArguments.cpp

@@ -132,6 +132,7 @@ class PreloadKernelArgInfo {
     NF->setAttributes(AL);
     F.replaceAllUsesWith(NF);
     F.setCallingConv(CallingConv::C);
+    F.clearMetadata();
 
     return NF;
   }

llvm/test/CodeGen/AMDGPU/preload-implicit-kernargs-debug-info.ll

@@ -1,7 +1,13 @@
-; RUN: not --crash opt -mtriple=amdgcn-amd-amdhsa -mcpu=gfx940 -passes='amdgpu-attributor,function(amdgpu-lower-kernel-arguments)' -amdgpu-kernarg-preload-count=16 -S < %s 2>&1 | FileCheck %s
+; RUN: opt -mtriple=amdgcn-amd-amdhsa -mcpu=gfx940 -passes='amdgpu-attributor,function(amdgpu-lower-kernel-arguments)' -amdgpu-kernarg-preload-count=16 -S < %s 2>&1 \
+; RUN: | FileCheck --match-full-lines --implicit-check-not='declare' %s
 
-; CHECK: function declaration may only have a unique !dbg attachment
-; CHECK-NEXT: ptr @0
+; Confirms we do not leave behind a declaration which references the same
+; DISubprogram metadata.
+
+; CHECK: define amdgpu_kernel void @preload_block_count_x{{.*}} !dbg ![[#]] !max_work_group_size ![[#]] {
+; CHECK: declare void @0{{.*}} #[[#]]
+; CHECK: declare noundef align 4 ptr addrspace(4) @llvm.amdgcn.implicitarg.ptr() #[[#]]
+; CHECK: declare noundef align 4 ptr addrspace(4) @llvm.amdgcn.kernarg.segment.ptr() #[[#]]
 
 define amdgpu_kernel void @preload_block_count_x(ptr addrspace(1) %out) !dbg !4 !max_work_group_size !7 {
   %imp_arg_ptr = call ptr addrspace(4) @llvm.amdgcn.implicitarg.ptr()