An alternative fix for PR70944

The transformations made by make_compound_operation apply
only to scalar integer modes.  The fix for PR70944 had enforced
that by returning early for vector modes at the top of the
function.  However, the function is supposed to be recursive,
so we should continue to look at integer suboperands even if
the outer operation is a vector one.

This patch instead splits out the non-recursive parts
of make_compound_operation into a subroutine and checks
that the mode is a scalar integer before calling it.
The patch was originally written to help with the later
conversion to static type checking of mode classes, but it
also happened to reenable optimisation of things like
vec_duplicate operands.

Note that the gen_lowparts in the PLUS and MINUS cases
were redundant, since new_rtx already had mode "mode"
at those points.

gcc/
2016-11-15  Richard Sandiford  <richard.sandiford@arm.com>
	    Alan Hayward  <alan.hayward@arm.com>
	    David Sherwood  <david.sherwood@arm.com>

	* combine.c (maybe_swap_commutative_operands): New function.
	(combine_simplify_rtx): Use it.
	(change_zero_ext): Likewise.
	(make_compound_operation_int): New function, split out of...
	(make_compound_operation): ...here.  Use
	maybe_swap_commutative_operands for both.

Co-Authored-By: Alan Hayward <alan.hayward@arm.com>
Co-Authored-By: David Sherwood <david.sherwood@arm.com>

From-SVN: r242492