xtensa: Improve bswap[sd]i2 insn patterns

xtensa: Improve bswap[sd]i2 insn patterns

This patch makes bswap[sd]i2 better register allocation, and reconstructs
bswapsi2 in order to take advantage of GIMPLE manual byte-swapping
recognition.

gcc/ChangeLog:

	* config/xtensa/xtensa.md (bswapsi2): New expansion pattern.
	(bswapsi2_internal): Revise the template and condition, and add
	detection code for preceding the same insn in order to omit a
	"SSAI 8" instruction of the latter.
	(bswapdi2): Suppress built-in insn expansion with the corresponding
	library call when optimizing for size.

gcc/testsuite/ChangeLog:

	* gcc.target/xtensa/bswap.c: Remove test.
	* gcc.target/xtensa/bswap-O1.c: New.
	* gcc.target/xtensa/bswap-O2.c: Ditto.
	* gcc.target/xtensa/bswap-Os.c: Ditto.