forwprop: Add LMK_INVALID and check for it

This adds some checks to make sure only a full initialized
long_mul_summand is added the vector.
The checks are only enabled for non release checking.

gcc/ChangeLog:

	* tree-ssa-forwprop.cc (enum long_mul_kind): Add LMK_INVALID.
	(long_mul_set_summand): Add default case to switch.
	(long_mul_summand_compare): Add checking to make sure
	the kinds are not LMK_INVALID.
	(long_mul_find_summand): Likewise.
	(long_mul_classify_chain): Add checking right before
	the push to check for LMK_INVALID.
	(match_long_mul_phi): Simpilify stynx for initializing carry.

Signed-off-by: Andrea Pinski <andrew.pinski@oss.qualcomm.com>