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

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

Patch by Milos Stojanovic.

Differential Revision: https://reviews.llvm.org/D33988

llvm-svn: 306095