Add type testing and casting support.

Add type testing and casting support.

Introduce a virtual tag() function to Instruction classes, use it to
implement type testing and casting.  Use type testing and casting to
eliminate some virtual functions on the instruction classes.

R=srdjan@google.com
BUG=
TEST=

Committed: https://code.google.com/p/dart/source/detail?r=4546

[Kevin Millikin (Google), 2012-02-23 13:17:53]

I hope this isn't premature optimization.  It seems nicer to get it in now when
there are relatively few classes and functions than to try to do it later.

1. I introduced a abstract base class for basic block entries.  I do think we
should keep the class hierarchy relatively flat.

2. I see from ast.h that it uses AsXXX-style (non-static) cast functions.  I do
prefer the static ones as here because I think it sometimes lets me avoid
redundant tests, but the AsXXX ones can be convenient too.  Do you have a strong
preference for the others?

[srdjan, 2012-02-23 16:50:26]

LGTM once addressing the comments and changing to IsXXX and AsXXX.

https://chromiumcodereview.appspot.com/9454012/diff/1/runtime/vm/intermediate...
File runtime/vm/intermediate_language.h (right):

https://chromiumcodereview.appspot.com/9454012/diff/1/runtime/vm/intermediate...
runtime/vm/intermediate_language.h:206: #undef DEFINE_TYPE_FUNCTIONS
I think the IsXXX and AsXXX are better. Typical use is:
a) test that something IsXXX
b) convert something with AsXXX

or

a) convert something with AsXXX and check if it is NULL.

The proposed design does not allow for second possibility. The first use
requires a virtual call for tag() anyway.
Yes I think eliminating virtual functions at this stage is premature
optimization. I think good design will set us well for future performance. Good
design would also include use of polymorphism instead of explicit type tests and
type conversion.

Sorry if I am sounding a little preachy here :-)

https://chromiumcodereview.appspot.com/9454012/diff/1/runtime/vm/intermediate...
runtime/vm/intermediate_language.h:252: intptr_t block_number_;
Optional: DISALLOW_yada_yada (here and below), but you ar right that it is not
needed since the superclass has a private one

https://chromiumcodereview.appspot.com/9454012/diff/1/runtime/vm/intermediate...
runtime/vm/intermediate_language.h:293: intptr_t block_number_;
Superclass has already field block_number_, remove it here and in constructor.