[TableGen] Add "getOperandType" to get operand types from opcode/opidx

The InstrInfoEmitter outputs an enum called "OperandType" which gives
numerical IDs to each operand type. This patch makes use of this enum
to define a function called "getOperandType", which allows looking up
the type of an operand given its opcode and operand index.

Patch by Nicolas Guillemot. Thanks!

Differential Revision: https://reviews.llvm.org/D63320

llvm-svn: 366274