ARM refactor indexed store instructions.

ARM refactor indexed store instructions.

Refactor STR[B] pre and post indexed instructions to use addressing modes for
memory operands, which is necessary for assembly parsing and is more consistent
with the rest of the memory instruction definitions. Make some incremental
progress on refactoring away the mega-operand addrmode2 along the way, which
is nice.

git-svn-id: https://llvm.org/svn/llvm-project/llvm/trunk@136978 91177308-0d34-0410-b5e6-96231b3b80d8