[mips] Add microMIPS specific addressing patterns.

These are identical but use microMIPS instructions instead of MIPS instructions.

Also, flatten the 'let AdditionalPredicates = [InMicroMips]' by using the
ISA_MICROMIPS adjective. Add tests for constant materialization.

Reviewers: atanasyan, abeserminji, smaksimovic

Differential Revision: https://reviews.llvm.org/D48275

llvm-svn: 335185