re PR libgomp/83590 ([nvptx] openacc reduction C regressions)

	PR libgomp/83590
	* gimplify.c (gimplify_one_sizepos): For is_gimple_constant (expr)
	return early, inline manually is_gimple_sizepos.  Make sure if we
	call gimplify_expr we don't end up with a gimple constant.
	* tree.c (variably_modified_type_p): Don't return true for
	is_gimple_constant (_t).  Inline manually is_gimple_sizepos.
	* gimplify.h (is_gimple_sizepos): Remove.

Co-Authored-By: Richard Biener <rguenther@suse.de>

From-SVN: r256748