[69/77] Split scalar-only part out of convert_mode

[69/77] Split scalar-only part out of convert_mode

This patch splits the final scalar-only part of convert_mode out
into its own subroutine and treats the modes as scalar_modes there.

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

gcc/
	* expr.c (convert_mode): Split scalar handling out into...
	(convert_mode_scalar): ...this new function.  Treat the modes
	as scalar_modes.

Co-Authored-By: Alan Hayward <alan.hayward@arm.com>
Co-Authored-By: David Sherwood <david.sherwood@arm.com>
From-SVN: r251520