tProc Peripheral instructions PA/PB hang if s6/s7 are used as source registers

[mmdiego]

Reported by Larry on Discord UF channel - https://discordapp.com/channels/764231928676089909/1404867577360027770

If either s6 or s7 registers are passed as arguments of a Peripheral instruction PA/PB, the tProc hangs it's execution. This seems to be a limitation on that instruction, as s6 and s7 are the registers where the peripheral instruction writes the return data. Because of this, tProc considers reading from s6 or s7 after a peripheral instruction as a Hazard and stalls the processor. However, in this case, it's the same instruction the one reading from those registers and produces a deadlock.

This should be flagged as a limitation of those instructions, or either analyze if it's worth it to allow the use of those registers.

[mmdiego]

I've also seen the problem occurs only if s6/s7 are used as A or B source registers, but not with C or D. Because that instruction can use 4 source registers (where most of the other instructions use only 2) the hazards block is only checking the registers on the "Data" section of the instruction (bits 38:23), but not the ones on the "Address" section (bits 50:39). This should be fixed somehow, either allowing all registers to use s6/s7 or not.