Add an is_narrower_int_mode helper function

Add an is_narrower_int_mode helper function

This patch adds a function for testing whether an arbitrary mode X
is an integer mode that is narrower than integer mode Y.  This is
useful for code like expand_float and expand_fix that could in
principle handle vectors as well as scalars.

2017-11-01  Richard Sandiford  <richard.sandiford@linaro.org>
	    Alan Hayward  <alan.hayward@arm.com>
	    David Sherwood  <david.sherwood@arm.com>

gcc/
	* machmode.h (is_narrower_int_mode): New function
	* optabs.c (expand_float, expand_fix): Use it.
	* dwarf2out.c (rotate_loc_descriptor): Likewise.

Co-Authored-By: Alan Hayward <alan.hayward@arm.com>
Co-Authored-By: David Sherwood <david.sherwood@arm.com>
From-SVN: r254305