Keep track of the FUNCTION_BEG note

function.cc emits a NOTE_FUNCTION_BEG after all arguments have
been copied to pseudos.  It then records this note in parm_birth_insn.
Various other pieces of code use this insn as a convenient place to
insert things at the start of the function.

However, cfgexpand later changes parm_birth_insn as follows:

  /* If we emitted any instructions for setting up the variables,
     emit them before the FUNCTION_START note.  */
  if (var_seq)
    {
      emit_insn_before (var_seq, parm_birth_insn);

      /* In expand_function_end we'll insert the alloca save/restore
	 before parm_birth_insn.  We've just insertted an alloca call.
	 Adjust the pointer to match.  */
      parm_birth_insn = var_seq;
    }

But the FUNCTION_BEG note is still useful for things that aren't
sensitive to stack allocation, and it has the advantage that
(unlike the var_seq above) it is never deleted or combined.
This patch adds a separate variable to track it.

gcc/
	* emit-rtl.h (rtl_data::x_function_beg_note): New member variable.
	(function_beg_insn): New macro.
	* function.cc (expand_function_start): Initialize function_beg_insn.

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