[mips] Process numeric register name in the .set assignment directive

Now LLVM assembler cannot process the following code and generates an
error. GNU tools support .set assignment directive with numeric register
name.

```
.set r4, 4

test.s:1:11: error: invalid token in expression
  .set r4, $4
           ^
```

This patch teach assembler to handle such directives correctly.
Unfortunately a numeric register name cannot be represented as an
expression. That's why we have to maintain a separate `StringMap`
in the `MipsAsmParser` to keep mapping between aliases names and
register numbers.

Differential revision: https://reviews.llvm.org/D47464

llvm-svn: 333428