vc4: Add separate write-after-read dependency tracking for pairing.

vc4: Add separate write-after-read dependency tracking for pairing.

If an operation is the last one to read a register, the instruction
containing it can also include the op that has the next write to that
register.

total instructions in shared programs: 57486 -> 56995 (-0.85%)
instructions in affected programs:     43004 -> 42513 (-1.14%)