I'd bet dollars to peanuts that the temporaries are stored in the gpr prf. It's big, and it's sized so it's not usually a constraint, so they can just push memory operands in there when there is free space and when it would be useful. And most importantly, unlike the vector registers (or any other structure already in the CPU), it can directly feed the ALUs without any new datapaths.
Probably physical register file as referenced by https://en.wikipedia.org/wiki/Register_file. PRF in this context refers to the actual memory that contains all of the register data, which can be much larger than the number of architectural registers to provide for register renaming, which can help performance (basically, the mapping from architectural register names to actual physical registers continually changes so you relieve the bottleneck of waiting for a particular physical register to become available).
PRF = Physical Register File. In modern CPUs, there are no separate architectural registers, all register state is held in a single very large register file (for example, 180 entries in Intel Skylake) that contains all register data, and the architectural register names are pointers into this file.
This is different from previous approaches where in-flight register data was held in the ROB and later written back into architectural registers.
