This commit is part of a PR that requires corresponding changes in SBY.
To prevent CI failures, detect whether those changes already landed and
skip the SBY using tests until then.
The previously generated logic assumed an unconstrained past value in
the initial state and did not handle 'x values. While the current formal
verification flow uses 2-valued logic, SVA value change expressions
require a past value of 'x during the initial state to behave in the
expected way (i.e. to consider both an initial 0 and an initial 1 as
$changed and an initial 1 as $rose and an initial 0 as $fell).
This patch now generates logic that at the same time
a) provides the expected behavior in a 2-valued logic setting, not
depending on any dont-care optimizations, and
b) properly handles 'x values in yosys simulation
For SVAs that have an explicit clock and are contained in a procedure
which conditionally executes the assertion, verific expresses this using
a mux with one input connected to constant 1 and the other output
connected to an SVA_AT. The existing code only handled the case where
the first input is connected to 1. This patch also handles the other
case.
