MIPS16: Simplify extended operand handling

MIPS16: Simplify extended operand handling

Simplify extended operand handling and only specially process immediates
which require bit shuffling, using the generic operand insertion and
extraction handlers for the '<' (5-bit shift amount) operand code in
particular.  Require the least significant bit of all extended operand
forms to be (artificially) set to 0 for their special processing to
trigger.

	gas/
	* config/tc-mips.c (mips16_immed): Limit `mips16_immed_extend'
	use to operands whose LSB position is zero.

	opcodes/
	* mips-dis.c (print_mips16_insn_arg): Simplify processing of
	extended operands.
	* mips16-opc.c (decode_mips16_operand): Switch the extended
	form of the `<' operand type to LSB position 22.

(cherry picked from commit bdd152861ce75c36828904cf3d10f8ce14da6cf5)