Unnecessary stalling on EX_rd / ID_(rs1/rs2) when rs2-value is actually an immediate

Instruction sequence
```
lw x4, 0(x0)
lw x5, 4(x0)
```
Causes a forwarding stall since the immediate in the second instruction (4) is registered as the rs2-value, even though the rs2-value is unused in LW instructions.

Solution(?): Attach flag that indicates whether rs1/rs2-values are used