icache: Improve latencies when reloading cache lines

icache: Improve latencies when reloading cache lines

The icache can now detect a hit on a line being refilled from memory,
as we have an array of individual valid bits per row for the line
that is currently being loaded.  This enables the request that
initiated the refill to be satisfied earlier, and also enables
following requests to the same cache line to be satisfied before the
line is completely refilled.  Furthermore, the refill now starts
at the row that is needed.  This should reduce the latency for an
icache miss.

We now get a 'sequential' indication from fetch1, and use that to know
when we can deliver an instruction word using the other half of the
64-bit doubleword that was read last cycle.  This doesn't make much
difference at the moment, but it frees up cycles where we could test
whether the next line is present in the cache so that we could
prefetch it if not.

Signed-off-by: Paul Mackerras <paulus@ozlabs.org>