Mips assembler: Improve set register alias handling

This patch solves the problem of numeric register values not being accepted:

../set_alias.s:1:11: error: expected valid expression after comma
        .set    r4,$4
                    ^
The parsing of .set directive is changed and handling of symbols in code 
as well to enable this feature. 

The test example is added.

Patch by Vladimir Medic


git-svn-id: https://llvm.org/svn/llvm-project/llvm/trunk@182807 91177308-0d34-0410-b5e6-96231b3b80d8