Before this commit, instructions were defined by a sequence of Const
for fixed fields (e.g. PO/XO) and AnyConst for others (e.g. operands).
This approach restricted their use to BMC use-cases, and prevented them
from appearing in VCD traces.
After this commit, an instruction encoding is defined by a Record. As
fields can now be set to arbitrary values, the corresponding InsnSpec
will only assert `pfv.stb` if `pfv.insn` matches a valid encoding (i.e.
fixed fields have correct values). On the other side, BMC testbenches
will drive `pfv.insn` with an AnyConst, and assume `pfv.stb` is high.
Jean-François Nguyen