Avoid visiting newly-created blocks in harden-conditionals

Avoid visiting newly-created blocks in harden-conditionals

Reverse iteration over blocks, in gimple-harden-conditionals.cc, was
supposed to avoid visiting blocks introduced by hardening and
introducing further reversed conditionals and traps for them, but
newly-created blocks may be inserted before the current block, as
shown by the PR105455 testcase.

Use an auto_sbitmap to gather preexisting blocks that need visiting.

for  gcc/ChangeLog

	* gimple-harden-conditionals.cc: Include sbitmap.h.
	(pass_harden_conditional_branches::execute): Skip new blocks.
	(pass_harden_compares::execute): Likewise.