[ftracer] Add caching of can_duplicate_bb_p

[ftracer] Add caching of can_duplicate_bb_p

The fix "[omp, ftracer] Don't duplicate blocks in SIMT region" adds iteration
over insns in ignore_bb_p, which makes it more expensive.

Counteract this by piggybacking the computation of can_duplicate_bb_p onto
count_insns, which is called at the start of ftracer.

Bootstrapped and reg-tested on x86_64-linux.

gcc/ChangeLog:

2020-10-05  Tom de Vries  <tdevries@suse.de>

	* tracer.c (count_insns): Rename to ...
	(analyze_bb): ... this.
	(cache_can_duplicate_bb_p, cached_can_duplicate_bb_p): New function.
	(ignore_bb_p): Use cached_can_duplicate_bb_p.
	(tail_duplicate): Call cache_can_duplicate_bb_p.