This patch is a partial implementation of mips .set assembler directive....

This patch is a partial implementation of mips .set assembler directive. Directive is defined as follows:
.set option
The patch implements following options

    at - lets the assembler use the $at register for macros,
         but generates warnings if the source program uses $at

    noat - let source programs use $at without issuingwarnings.

    noreorder - prevents the assembler from reordering machine 
                language instructions.
    nomacro - causes the assembler to print a warning whenever 
              an assembler operation generates more than one 
              machine language instruction.
    macro - lets the assembler generate multiple machine instructions 
            from a single assembler instruction
    reorder - lets the assembler reorder machine language 
               instructions to improve performance

The above variants are parsed and their boolean values set or unset.
The code to actually use them will come later.

Following options are not implemented yet:

nomips16
nomicromips
move
nomove

Contributer: Vladimir Medic
llvm-svn: 165194