This patch enables the Mips .set directive to define aliases

The .set directive in the Mips the assembler can be 
used to set the value of a symbol to an expression. 
This changes the symbol's value and type to conform 
to the expression's.

Syntax: .set symbol, expression

This patch implements the parsing of the above syntax 
and enables the parser to use defined symbols when 
parsing operands.

Contributor: Vladimir Medic
llvm-svn: 177667