Mips assembler: Add TwoOperandConstraint definitions

This patch removes alias definition for addiu $rs,$imm 
and instead uses the TwoOperandAliasConstraint field in 
the ArithLogicI instruction class. 

This way all instructions that inherit ArithLogicI class 
have the same macro defined. 

The usage examples are added to test files.

Patch by Vladimir Medic

llvm-svn: 182048