[mips] Fix the definitions of lwp, swp

Rather than using a regpair operand of these instructions, use two seperate
operands and a custom converter to handle the implicit second register operand.

Additionally, remove the microMIPS32R6 definition as its redundant.

Reviewers: atanasyan, abeserminji, smaksimovic

Differential Revision: https://reviews.llvm.org/D47255

llvm-svn: 333288