Fix RegType invariant checks.

Move the checks from the base constructor to constructors of
the most derived classes. While in the base constructor, the
vtable points to the base class implementations of virtual
function, so we were not checking what we intended. And when
the check failed, we tried to call a pure virtual function
RegType::Dump() and crashed while constructing an abort
message.

Also, avoid an unnecessary std::string construction in
RegType::CheckInvariants() as StringPiece can be compared
with const char* directly.

Test: testrunner.py --host
Bug: 32548008
Change-Id: Ice7ca298132f87f8b1671b6578aeec221f0e7930