LoongArch: Fix the semantic of 16B CAS

LoongArch: Fix the semantic of 16B CAS

In a CAS operation, even if expected != *memory we still need to do an
atomic load of *memory into output.  But I made a mistake in the initial
implementation, causing the output to contain junk in this situation.

Like a normal atomic load, the atomic load embedded in the CAS semantic
is required to work on read-only page.  Thus we cannot rely on sc.q to
ensure the atomicity of the load.  Use LSX to perform the load instead,
and also use LSX to compare the 16B values to keep the ll-sc loop body
short.

gcc/ChangeLog:

	* config/loongarch/sync.md (atomic_compare_and_swapti_scq):
	Require LSX.  Change the operands for the output, the memory,
	and the expected value to LSX vector modes.  Add a FCCmode
	output to indicate if CAS has written the desired value into
	memory.  Use LSX to atomically load both words of the 16B value
	in memory.
	(atomic_compare_and_swapti): Pun the modes to satisify
	the new atomic_compare_and_swapti_scq implementation.  Read the
	bool return value from the FCC instead of performing a
	comparision.