MIPS16: Fix SP-relative SD instruction annotation

MIPS16: Fix SP-relative SD instruction annotation

Fix the annotation of SP-relative SD instructions incorrectly marked as
reading from the PC rather than SP, which in turn prevented their 16-bit
forms from being scheduled into jump delay slots.  This bug has been
there since forever.

	opcodes/
	* mips16-opc.c (mips16_opcodes): Set RD_SP rather than RD_PC in
	`pinfo2' with SP-relative "sd" entries.

	gas/
	* testsuite/gas/mips/mips16-sprel-swap.d: New test.
	* testsuite/gas/mips/mips16-sprel-swap.s: New test source.
	* testsuite/gas/mips/mips.exp: Run the new test.