[74/77] Various small scalar_mode changes

This patch uses scalar_mode in a few miscellaneous places:

- Previous patches mean mode_to_vector can take a scalar_mode without
  further changes.

- Implicit promotion is limited to scalar types (affects promote_mode
  and sdbout_parms)

2017-08-30  Richard Sandiford  <richard.sandiford@linaro.org>
	    Alan Hayward  <alan.hayward@arm.com>
	    David Sherwood  <david.sherwood@arm.com>

gcc/
	* machmode.h (mode_for_vector): Take a scalar_mode instead
	of a machine_mode.
	* stor-layout.c (mode_for_vector): Likewise.
	* explow.c (promote_mode): Use as_a <scalar_mode>.
	* sdbout.c (sdbout_parms): Use is_a <scalar_mode>.

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

From-SVN: r251525