target/108738 - optimize bit operations in STV

target/108738 - optimize bit operations in STV

The following does low-hanging optimizations, combining bitmap
test and set and removing redundant operations.

	PR target/108738
	* config/i386/i386-features.cc (scalar_chain::add_to_queue):
	Combine bitmap test and set.
	(scalar_chain::add_insn): Likewise.
	(scalar_chain::analyze_register_chain): Remove redundant
	attempt to add to queue and instead strengthen assert.
	Sink common attempts to mark the def dual-mode.
	(scalar_chain::add_to_queue): Remove redundant insn bitmap
	check.