[53/77] Pass a mode to const_scalar_mask_from_tree

[53/77] Pass a mode to const_scalar_mask_from_tree

The caller of const_scalar_mask_from_tree has proven that
the mode is a MODE_INT, so this patch passes it down as a
scalar_int_mode.  It also expands the comment a little.

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

gcc/
	* expr.c (const_scalar_mask_from_tree): Add a mode argument.
	Expand commentary.
	(expand_expr_real_1): Update call accordingly.

git-svn-id: svn+ssh://gcc.gnu.org/svn/gcc/trunk@251505 138bc75d-0d04-0410-961f-82ee72b054a4