[TableGen] Add support for negative immediates to AsmMatcherEmitter

This adds support for creating an InstAlias with a negative immediate, i.e.:

  def NOT : InstAlias<"not $dst, $src", (XORI GR32:$dst, GR32:$src, -1)>;

by resolving this problem:

RISCVGenAsmMatcher.inc:95:11: error: expected '= constant-expression' or end of enumerator definition
  CVT_imm_-1,
  ^^^^^^^^^^

Patch by Jordy Potman, thanks!

llvm-svn: 226073