Add MLA alias for ARMv4 support.

Fix MLA defs to use register class GPRnopc.
Add encoding tests for multiply instructions.
(Alias for MUL/SMLAL/UMLAL added by r199026.)

Patch by Zhaoshi.

git-svn-id: https://llvm.org/svn/llvm-project/llvm/trunk@199491 91177308-0d34-0410-b5e6-96231b3b80d8