Patch to implement rotates by zero

[Eric Christopher]

> 
> OK.  Here's a revised patch.  Tested as before on mips-elf.
> 
> Richard
> 
> 
> [gas/]
> 	* config/tc-mips.c (macro2): Implement rotates by zero using shifts
>         by zero.
> 
> [gas/testsuite]
> 	* gas/mips/rol.s: Add rotate by zero tests.
> 	* gas/mips/rol.d: Update accordingly.
> 	* gas/mips/rol64.d: Expect rotates by zero to use dsrl.
> 

Looks cool to me.

-eric

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