[58/77] Use scalar_int_mode in a try_combine optimisation

This patch uses scalar_int_modes for:

  /* If I2 is setting a pseudo to a constant and I3 is setting some
     sub-part of it to another constant, merge them by making a new
     constant.  */

This was already implicit, but the danger with checking only
CONST_SCALAR_INT_P is that it can include CC values too.

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

gcc/
	* combine.c (try_combine): Use is_a <scalar_int_mode> when
	trying to combine a full-register integer set with a subreg
	integer set.

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

From-SVN: r251510