ifcvt: cond zero arith: opencode helper noce_bbs_ok_for_cond_zero_arith [NFC]

ifcvt: cond zero arith: opencode helper noce_bbs_ok_for_cond_zero_arith [NFC]

This makes the code more readable by eliminating a bunch of pointer
intermediaries which obfuscate if_info items needed later in
noce_try_cond_zero_arith (). And while here add some top level comments
about what cond zero arith actually does.

gcc/ChangeLog:

	* ifcvt.cc (noce_bbs_ok_for_cond_zero_arith): Move logic out.
	(noce_try_cond_zero_arith): Into here.

Signed-off-by: Vineet Gupta <vineetg@rivosinc.com>