[mips] divide macro instruction cleanup.

Clean up the implementation of divide macro expansion by getting rid of a
FIXME regarding magic numbers and branch instructions. Match GAS' behaviour
for expansion of ddiv / div in the two and three operand cases. Add the two
operand alias for MIPSR6. Finally, optimize macro expansion cases where the
divisior is the $zero register.

Reviewers: slthakur

Differential Revision: https://reviews.llvm.org/D29887

llvm-svn: 294960