[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


git-svn-id: https://llvm.org/svn/llvm-project/llvm/trunk@333288 91177308-0d34-0410-b5e6-96231b3b80d8
5 files changed