Thumb2 parsing and encoding for LDMIA.

Thumb2 parsing and encoding for LDMIA.

Choose 32-bit vs. 16-bit encoding when there's no .w suffix in post-processing
as match classes are insufficient to handle the context-sensitiveness of
the writeback operand's legality for the 16-bit encodings.

git-svn-id: https://llvm.org/svn/llvm-project/llvm/trunk@139242 91177308-0d34-0410-b5e6-96231b3b80d8