This patch enables the Mips .set directive to define aliases

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

git-svn-id: https://llvm.org/svn/llvm-project/llvm/trunk@177667 91177308-0d34-0410-b5e6-96231b3b80d8