Use can_open_code_p in gimple-ssa-store-merging

...and extend can_open_code_p to handle bswap.

I'm not sure that the removed builtin_decl_explicit_p (BUILT_IN_BSWAP32)
calls are necessary, since the code would go on to use a BSWAP64
rather than a BSWAP32.

This fixes gcc.dg/optimize-bswapsi-6.c on RISC-V.

gcc/
	* optabs-query.cc (can_open_code_p): Handle more bswap cases,
	incorporating logic from...
	* gimple-ssa-store-merging.cc (maybe_optimize_vector_constructor)
	(pass_optimize_bswap::execute)
	(imm_store_chain_info::try_coalesce_bswap): ...here.  Use
	can_open_code_p instead of direct optab_handler checks.