Make more use of GET_MODE_UNIT_BITSIZE

Make more use of GET_MODE_UNIT_BITSIZE

This patch is like the previous GET_MODE_UNIT_SIZE one,
but for bit rather than byte sizes.

2017-10-13  Richard Sandiford  <richard.sandiford@linaro.org>
	    Alan Hayward  <alan.hayward@arm.com>
	    David Sherwood  <david.sherwood@arm.com>

gcc/
	* cfgexpand.c (expand_debug_expr): Use GET_MODE_UNIT_BITSIZE.
	(expand_debug_source_expr): Likewise.
	* combine.c (combine_simplify_rtx): Likewise.
	* cse.c (fold_rtx): Likewise.
	* fwprop.c (canonicalize_address): Likewise.
	* targhooks.c (default_shift_truncation_mask): Likewise.

Co-Authored-By: Alan Hayward <alan.hayward@arm.com>
Co-Authored-By: David Sherwood <david.sherwood@arm.com>
From-SVN: r253716