Refactor the instructions into fixed writeback and register-stride
writeback variants to simplify the offset operand (no more optional
register operand using reg0). This is a simpler representation and allows
the assembly parser to more easily handle these instructions.
Add tests for the instruction variants now supported.
git-svn-id: https://llvm.org/svn/llvm-project/llvm/trunk@146278 91177308-0d34-0410-b5e6-96231b3b80d8