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
69301c9eb9