MIPS/gas: Fix order of instructions in LI macro expansion

MIPS/gas: Fix order of instructions in LI macro expansion

When MTHC1 instruction is paired with MTC1 to write a value to a
64-bit FPR, the MTC1 must be executed first, because the semantic
definition of MTC1 is not aware that software will be using an MTHC1
to complete the operation, and sets the upper half of the 64-bit FPR
to an UNPREDICTABLE value[1].

Fix the order of MTHC1 and MTC1 instructions in LI macro expansion.
Modify the expansions to exploit moves from $zero directly by-passing
the use of $AT, where ever possible.

[1] "MIPS Architecture for Programmers Volume II-A: The MIPS32
     Instruction Set Manual", Wave Computing, Inc., Document
     Number: MD00086, Revision 5.04, December 11, 2013, Section 3.2
     "Alphabetical List of Instructions", pp. 217.

gas/
	* config/tc-mips.c (macro) <M_LI>: Re-order MTHC1 with
	respect to MTC1 and use $0 for either part where possible.
	* testsuite/gas/mips/li-d.s: Add test cases for non-zero
	words in double precision constants.
	* testsuite/gas/mips/li-d.d: Update reference output.
	* testsuite/gas/mips/micromips@isa-override-1.d: Likewise.
	* testsuite/gas/mips/mips32r2@isa-override-1.d: Likewise.
	* testsuite/gas/mips/mips64r2@isa-override-1.d: Likewise.