Method resolveInFlow

For certain data-types, particularly \e union, variables of that data-type are transformed into a subtype depending on the particular use. Each read or write of the variable may use a different subtype. This method returns the particular subtype required based on a specific PcodeOp. A slot index >=0 indicates which operand \e reads the variable, or if the index is -1, the variable is \e written.