MIPS16/GAS: Improve non-constant operand error diagnostics

MIPS16/GAS: Improve non-constant operand error diagnostics

Improve operand error diagnostics for non-constant expressions used for
a 16-bit immediate, making the message more descriptive and indicating
the offending operand, e.g.:

foo.s:1: Error: invalid operands `lui $2,foo-bar'

will show as:

foo.s:1: Error: operand 2 must be constant `lui $2,foo-bar'

This case does not currently trigger however, for two reasons.

First, for regular MIPS and microMIPS assembly in the case of no match
caused by `match_int_operand' here, the function is always called again
from `mips_ip' via `match_insns', `match_insn' and then `match_operand'
for the same opcode table's entry with `lax_match' set to TRUE, in which
case the attempt to match succeeds and no error is issued.

Second, in the case of MIPS16 assembly no call to `match_int_operand' is
made at all for 16-bit immediates, because such immediates are currently
only matched with extensible instructions, and these are handled in
`match_mips16_insn' via `match_expression' directly rather than via
`match_operand'.

This will change for MIPS16 code with MIPS16e2 support introduced, where
non-extensible instructions accepting 16-bit immediates will be added,
so make the case work well right from the start.

	gas/
	* config/tc-mips.c (match_int_operand): Call
	`match_not_constant' before returning failure for a non-constant
	16-bit immediate conditionally allowed.