icache: Reduce metavalue warnings

icache: Reduce metavalue warnings

As in dcache, this changes most signals declared with integer type to
be unsigned bit vectors instead.  Some code has been rearranged to do
to_integer() or equality comparisons only when the relevant signals
should be well defined.  Non-fatal asserts have been sprinkled
throughout to assist with determining the cause of warnings from
library functions (primarily NUMERIC_STD.TO_INTEGER and
NUMERIC_STD."=").

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