MIPS16/opcodes: Annotate instruction aliases

Complement commit 986e18a5a9fd ("Add a second 'pinfo' member to
mips_opcode to extend number of available bits"),
<https://sourceware.org/ml/binutils/2005-01/msg00261.html>, and annotate
MIPS16 NOP, LA, DLA and the synthetic forms of LD and LW instructions as
aliases.  These correspond to MOVE, and the PC-relative ADDIU, DADDIU,
LD and LW hardware instructions respectively.

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

	opcodes/
	* mips16-opc.c (AL): New macro.
	(mips16_opcodes): Mark "nop", "la", "dla", and synthetic forms
	of "ld" and "lw" as aliases.