[TableGen][CodeEmitterGen] Fix SubOpAliases MIOperandNo mixup

SubOpAliases maps a sub-operand name to the respective operand's index
and the sub-operand number within this operand. The operand index is
used for the Operands array.

Currently MIOperandNo is used as the operand index, which is not
correct. For example, if there are 2 operands with 3 sub-operands each:

  (ins (bdladdr12onlylen4 $B1, $D1, $L1):$BDL1,
       (bdladdr12onlylen4 $B2, $D2, $L2):$BDL2)

then B2's operand index will be 3, but the correct value is 1.

Reviewed By: jyknight

Differential Revision: https://reviews.llvm.org/D155158