This commit adds a new "discarded" state for registers.
Discarding a register is like flushing it, but without
actually writing its value back to memory. We can discard
a register only when it is guaranteed that no instruction
will read from the register before it is next written to.
Discarding reduces the register pressure a little, and can
also let us skip a few flushes on interpreter fallbacks.
JosJuice
62ce1c7653