MIPS/opcodes: Fix undecoded MIPS16 extended instruction bit disassembly

MIPS/opcodes: Fix undecoded MIPS16 extended instruction bit disassembly

Correct the disassembly of hardware don't cares in MIPS16 extended
instructions.  Rather than e.g.:

   0:	f008 0231 	addiu	v0,sp,16433
   4:	f520 3260 	sll	v0,v1,-12

print:

   0:	f008 0231 	addiu	v0,sp,16401
   4:	f520 3260 	sll	v0,v1,20

respectively instead.

	opcodes/
	* mips-dis.c (print_mips16_insn_arg): Mask unused extended
	instruction bits out.

	binutils/
	* testsuite/binutils-all/mips/mips16-undecoded.d: New test.
	* testsuite/binutils-all/mips/mips16-undecoded.s: New test
	source.
	* testsuite/binutils-all/mips/mips.exp: Run the new test.