[5/77] Small tweak to array_value_type

[5/77] Small tweak to array_value_type

Store the type mode in a variable so that a later,
more mechanical patch can change its type.

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

gcc/
	* tree-switch-conversion.c (array_value_type): Only read TYPE_MODE
	once.  Use get_narrowest_mode instead of GET_CLASS_NARROWEST_MODE.

Co-Authored-By: Alan Hayward <alan.hayward@arm.com>
Co-Authored-By: David Sherwood <david.sherwood@arm.com>
From-SVN: r251456