[AMDGPU] Fix debug line table for MSG_DEALLOC_VGPRS optimization (#88924)

Deallocating VGPRs interferes with doing a context save, which is needed for GDB
to report a breakpoint. So, in this sequence:

  s_sendmsg MSG_DEALLOC_VGPRS
  s_endpgm

We now use the debug location of the s_endpgm for the s_sendmsg, so a breakpoint
set in the debugger at the end of a shader will be hit before deallocating VGPRs.