[nvptx] Rewrite nvptx_goacc_validate_dims to use predicate vars

[nvptx] Rewrite nvptx_goacc_validate_dims to use predicate vars

The function nvptx_goacc_validate_dims has arguments decl and fn_level which
together describe different situations.

Introduce a predicate var for each situation, and use them, allowing to
understand what the function does in each situation without having to know the
way the situations are encoded in the args.

Build and reg-tested on x86_64 with nvptx accelerator.

2018-12-17  Tom de Vries  <tdevries@suse.de>

	* config/nvptx/nvptx.c (nvptx_goacc_validate_dims): Rewrite using
	predicate vars.

From-SVN: r267212