It was the caller's responsibility to call TargetIsa::check() before;
now one can't manipulate a TargetIsa without calling the
TargetIsaBuilder::finish() method, which is less error-prone and more in
line with what's coming for other things we're going to generate in the
meta crate.
Also splits the construction of a RegClass in two parts: a prototype is
made first when declaring the RegClass, and missing bits are filled in
when adding it to the TargetIsa(Builder). This avoids an awkward passing
of the isa to the RegClass ctor.
