MIPS/gas: Reject $0 as source register for DAUI instruction

MIPS/gas: Reject $0 as source register for DAUI instruction

The MIPS64R6 TRM requires that the source register for DAUI
not be r0.

[1] "MIPS Architecture for Programmers Volume II-A: The MIPS64
    Instruction Set Manual", Imagination Technologies Ltd., Document
    Number: MD00087, Revision 6.06, December 15, 2016, Section 3.2
    "Alphabetical List of Instructions", pp. 67-68.

gas/
	* testsuite/gas/mips/r6-branch-constraints.s: Rename to ...
	* testsuite/gas/mips/r6-reg-constraints.s: this and add test
	case for DAUI.
	* testsuite/gas/mips/r6-branch-constraints.l: Rename to ...
	* testsuite/gas/mips/r6-reg-constraints.l: this and add test
	for DAUI.
	* testsuite/gas/mips/mips.exp: Rename test from
	r6-branch-constraints to r6-reg-constraints.

opcodes/
	* mips-opc.c (mips_builtin_opcodes): Change source register
	constraint for DAUI.