mips: fix unintialized operand use in sync_{old,new}_<optab>_12 [PR 125628]

In GCC, if the RTL template of define_insn has multiple elements, it's
treated as a parallel expression.  And, "in parallel" means that first
all the values used in the invidiviual side-effects are computed, and
second all the actual side-effects are performed.  So when the value of
operand 1 (the output reg) is used, it's not set yet.

When optimization is enabled, the uninitialized value is replaced with 0
and then for e.g. if atomic_hiqi_op is plus, (plus (0) (val)) is folded
to simply (val).  Now the RTL template happens to be matched by
sync_old_nand_12 (of which the RTL is written in a really inconsistent
way), causing "0 + 1 = -1".

So fix the uninitialized operand use, i.e. (match_dup 0) should be
(match_dup 1).  Also slightly alter the source of the set for the memory
in sync_new_<optab>_12 to make it clear the value in the reg and in the
memory should be same after the operation.

gcc/

	PR target/125628
	* config/mips/sync.md (sync_old_<optab><mode>): Fix
	uninitialized operand use.
	(sync_new_<optab><mode>): Fix uninitialized operand use, use the
	same expression for the set source of operand 0 and 1.