[mips] Disable the selection of mixed microMIPS/MIPS code

This patch modifies hasStandardEncoding() / inMicroMipsMode() /
inMips16Mode() methods of the MipsSubtarget class so only one can be
true at any one time. That prevents the selection of microMIPS and MIPS
instructions and patterns that are defined in TableGen files at the same
time. A few new patterns and instruction definitions hae been added to
keep test cases passed.

Differential revision: https://reviews.llvm.org/D51483

llvm-svn: 341338