[ms-inline asm] Add the convertToMapAndConstraints() function that is used to

map constraints and MCInst operands to inline asm operands.  This replaces the
getMCInstOperandNum() function.

The logic to determine the constraints are not in place, so we still default to
a register constraint (i.e., "r"). Also, we no longer build the MCInst but
rather return just the opcode to get the MCInstrDesc.

llvm-svn: 164979