[FP] Remove `FLOATINGPOINT_TO_FP_GENERIC` kind (#8334)

[FP] Remove `FLOATINGPOINT_TO_FP_GENERIC` kind  (#8334)

This commit removes the `FLOATINGPOINT_TO_FP_GENERIC` kind, which was
only used in the parser and immediately rewritten by the floating-point
arithmetic solver. It extends the `ParseOp` class to handle indexed
operators of which we do not know the kind (`to_fp` in this case) by
storing the name and the indices. The name is then resolved later when
we have parsed the arguments.