Allow frame pointer elimination in 64-bit mips16 code

[Eric Christopher]

> mipsisa64-elf & mips64{,el}-elf.  OK to install?
> 
> Richard
> 
> 
> 	* config/mips/mips.h (EXTRA_CONSTRAINT): Add 'W' constraint.
> 	(EXTRA_MEMORY_CONSTRAINT): Define.
> 	(CAN_ELIMINATE): Remove lwu workaround.
> 	* config/mips/mips.md (*zero_extendsidi2_mem): Enable for mips16 too.
> 	Use a 'W' constraint for the source operand.

OK.

-eric

-- 
Eric Christopher <echristo@redhat.com>