Change operand list to n src fields

Simple tweak to change the phi functions from being atypical, having an "operands" field containing a list of all operators, to being more like other operations simply using an arbitrary number of "src" fields. The src fields are strictly ordered in the same order that the list would have been in the old implementation.

Sorry for the noisy diff, looks like I accidentally stripped all trailing spaces at some point in the process. May make a sneaky push to the tossa branch to fix that.