[mips] Rewrite MipsAsmParser and MipsOperand.

Summary:
Highlights:
- Registers are resolved much later (by the render method).
  Prior to that point, GPR32's/GPR64's are GPR's regardless of register
  size. Similarly FGR32's/FGR64's/AFGR64's are FGR's regardless of register
  size or FR mode. Numeric registers can be anything.
- All registers are parsed the same way everywhere (even when handling
  symbol aliasing)
  - One consequence is that all registers can be specified numerically
    almost anywhere (e.g. $fccX, $wX). The exception is symbol aliasing
    but that can be easily resolved.
- Removes the need for the hasConsumedDollar hack
- Parenthesis and Bracket suffixes are handled generically
- Micromips instructions are parsed directly instead of going through the
  standard encodings first.
- rdhwr accepts all 32 registers, and the following instructions that previously
  xfailed now work:
    ddiv, ddivu, div, divu, cvt.l.[ds], se[bh], wsbh, floor.w.[ds], c.ngl.d,
    c.sf.s, dsbh, dshd, madd.s, msub.s, nmadd.s, nmsub.s, swxc1
- Diagnostics involving registers point at the correct character (the $)
- There's only one kind of immediate in MipsOperand. LSA immediates are handled
  by the predicate and renderer.

Lowlights:
- Hardcoded '$zero' in the div patterns is handled with a hack.
  MipsOperand::isReg() will return true for a k_RegisterIndex token
  with Index == 0 and getReg() will return ZERO for this case. Note that it
  doesn't return ZERO_64 on isGP64() targets.
- I haven't cleaned up all of the now-unused functions.
  Some more of the generic parser could be removed too (integers and relocs
  for example).
- insve.df needed a custom decoder to handle the implicit fourth operand that
  was needed to make it parse correctly. The difficulty was that the matcher
  expected a Token<'0'> but gets an Imm<0>. Adding an implicit zero solved this.

Reviewers: matheusalmeida, vmedic

Reviewed By: matheusalmeida

Differential Revision: http://llvm-reviews.chandlerc.com/D3222