MIPS16/GAS: Clean up invalid unextended operand handling

MIPS16/GAS: Clean up invalid unextended operand handling

Bail out right away when an unextended instruction encoding is required
either with the use of a `.t' suffix or by means of `.set noautoextend',
however an operand supplied requires the extended instruction form to be
used.

This is to avoid messing up with the internal state of the assembler,
even though no actual failures are known to happen as a result.  Add
test cases for the situation concerned.

	gas/
	* config/tc-mips.c (match_mips16_insn): Don't update
	`forced_insn_length' or the instruction opcode if an operand
	requires an extended instruction form, but an unextended one
	has been requested.
	* testsuite/gas/mips/mips16-relax-unextended-1.d: New test.
	* testsuite/gas/mips/mips16-relax-unextended-2.d: New test.
	* testsuite/gas/mips/mips16-relax-unextended-1.l: New stderr
	output.
	* testsuite/gas/mips/mips16-relax-unextended-2.l: New stderr
	output.
	* testsuite/gas/mips/mips16-relax-unextended-1.s: New test
	source.
	* testsuite/gas/mips/mips16-relax-unextended-2.s: New test
	source.
	* testsuite/gas/mips/mips.exp: Run the new tests.