Reland r306095: [mips] Fix reg positions in the aui/daui instructions

After fixing (r306173) a failing test in the lld test suite (r306173),
reland r306095.

Original commit message:

  [mips] Fix register positions in the aui/daui instructions

  Swapped the position of the rt and rs register in the aui/daui
  instructions for mips32r6 and mips64r6. With this change, the format of
  the generated instructions complies with specifications and GCC.
  Patch by Milos Stojanovic.

llvm-svn: 306174