store-merging: Match bswap64 on 32-bit targets with bswapsi2 [PR114319]

gimple-ssa-store-merging.cc tests bswap_optab in 3 different places,
in 2 of them it has special exception for double-word bswap using pair
of word-mode bswap optabs, but in the last one it doesn't.

The following patch changes even the last spot.
We don't handle 128-bit bswaps in the passes at all, because currently we
just use uint64_t to represent the byte reshuffling (we'd need to use
offset_int or something like that instead) and we don't have
__builtin_bswap128 nor type-generic __builtin_bswap, so there is nothing
for 64-bit targets there.

2024-03-13  Jakub Jelinek  <jakub@redhat.com>

	PR middle-end/114319
	* gimple-ssa-store-merging.cc
	(imm_store_chain_info::try_coalesce_bswap): For 32-bit targets
	allow matching __builtin_bswap64 if there is bswapsi2 optab.

	* gcc.target/i386/pr114319.c: New test.