Prevent frontend deadlock fetching from uncacheable memory

After detecting a corrupted BTB, don't speculatively update it until
the next non-speculative fetch.  This prevents the frontend from replaying
forever.

Add cover points related to memory error to I/D Cache

* When writing full words to ITIM, ECC errors are correctable

* Disable D$ tag reset state machine when using scratchpad

Fix typo in breakpoint cover property.

The reason for the :=? operator was for when you have an adapter chain
whose direction of cardinality you could not know. We used explicit
directives to tell these compositions which way to go.

Unfortunately, that makes the API leaky. You think the chain of adapters
is just one adapter, but you have to use strange Cardinality scopes to
use it. That's just bad.

The new :*=* just automagically figures it out from the graph.

Added coverage point to cover the case when ECC error happens during …

In situ unit tests

This removes the mostly obsolete 'numIn/Out' range restrictions on nodes.
It also makes it possible to connect optional crossbars that disappear.

val x = TLXbar()
x := master
slave := x

val y = TLXbar()
x :=* y // only connect y if it gets used

This will create crossbar x, but crossbar y will disappear.

The new rule is you should have an object.apply method if you only have a
single .node.

This is necessary capture the node implementation in the handle,
which is in turn necessary to support cloning a Node.

TileLink compliance: d_bits may not depend on d_ready

RegField.bytes updates only those bytes which are written every cycle.
However, there was a bug that it would try to return the updated value on reads.
This led to another TL-spec violating combinational path, just like the Debug module.

When passed a Wire, WNotify outputs that wire on reads wire => d_bits.
Furthermore, it updates the Wire when a write occures d_ready => wire.

These registers should be returning undefined value on read, anyway.

Add cover properties for exceptions in the core.

Avoid data corruption under correctable tag error during flush

This esoteric bug manifests if a tag-read error occurs when a FENCE.I is
executed, even if the error was correctable.  Subsequently, an attempt to
flush a dirty line may flush the wrong line's data.

This reverts commit 44eb4d12.

This esoteric bug manifests if a tag-read error occurs when a FENCE.I is
executed, even if the error was correctable.  Subsequently, an attempt to
flush a dirty line may flush the wrong line's data.

debug: Remove workaround for Chisel 3 #527