Add must_pass_va_arg_in_stack

Add must_pass_va_arg_in_stack

This patch splits out another idiom from the va_arg gimplification
routines, so that there's only one place to update later.

2019-08-20  Richard Sandiford  <richard.sandiford@arm.com>

gcc/
	* calls.h (must_pass_va_arg_in_stack): Declare.
	* calls.c (must_pass_va_arg_in_stack): New function.
	* config/alpha/alpha.c (alpha_gimplify_va_arg_1): Use it.
	* config/sh/sh.c (sh_gimplify_va_arg_expr): Likewise.
	* config/stormy16/stormy16.c (xstormy16_gimplify_va_arg_expr):
	Likewise.
	* config/xtensa/xtensa.c (xtensa_gimplify_va_arg_expr): Likewise.

From-SVN: r274696