[gimple] Move can_duplicate_bb_p to gimple_can_duplicate_bb_p

[gimple] Move can_duplicate_bb_p to gimple_can_duplicate_bb_p

The function gimple_can_duplicate_bb_p currently always returns true.

The presence of can_duplicate_bb_p in tracer.c however suggests that
there are cases when bb's indeed cannot be duplicated.

Move the implementation of can_duplicate_bb_p to gimple_can_duplicate_bb_p.

Bootstrapped and reg-tested on x86_64-linux.

Build x86_64-linux with nvptx accelerator and tested libgomp.

No issues found.

As corner-case check, bootstrapped and reg-tested a patch that makes
gimple_can_duplicate_bb_p always return false, resulting in
PR97333 - "[gimple_can_duplicate_bb_p == false, tree-ssa-threadupdate]
ICE in duplicate_block, at cfghooks.c:1093".

gcc/ChangeLog:

2020-10-09  Tom de Vries  <tdevries@suse.de>

	* tracer.c (cached_can_duplicate_bb_p, analyze_bb): Use
	can_duplicate_block_p.
	(can_duplicate_insn_p, can_duplicate_bb_no_insn_iter_p)
	(can_duplicate_bb_p): Move and merge ...
	* tree-cfg.c (gimple_can_duplicate_bb_p): ... here.