Thumb assembly parsing and encoding for LDM instruction.

Thumb assembly parsing and encoding for LDM instruction.

Fix base register type and canonicallize to the "ldm" spelling rather than
"ldmia." Add diagnostics for incorrect writeback token and out-of-range
registers.

git-svn-id: https://llvm.org/svn/llvm-project/llvm/trunk@137986 91177308-0d34-0410-b5e6-96231b3b80d8