mips gas: expression initialisation

[Maciej W. Rozycki]

On Tue, 16 Sep 2025, Alan Modra wrote:

> There is a make_expr_symbol in append_insn, which gets called from
> macro_build, which is all over the place.  Many of these set up an
> expression without initialising all fields.  Now the uninitialised
> fields should not be accessed in a properly functioning assembler,
> but I'm inclined to think anything copied ought to be initialised.

 Thank you for looking into it.  I like your change except for the bits 
around `micromips_label_expr', which I find messy as they stand and at 
best no better with your modifications.

 I think this part ought to be a separate patch as the first step:

static expressionS
mips_label_expr (offsetT offset)
{
  return (expressionS) {
    .X_op = O_constant,
    .X_add_number = offset
  };
}

static expressionS
micromips_label_expr (void)
{
  return (expressionS) {
    .X_op = O_symbol,
    .X_add_symbol = symbol_find_or_make (micromips_label_name ())
  };
}

(unsure about correct indentation here), and then we'll get rid of all the 
mess and let the compiler figure out whether inlining the initialiser will 
be cheaper in terms of the optimisation level chosen or not.  WDYT?

  Maciej