Simplify ExtendsClass, InstanceOf, and InstanceOfIface

Summary: These cases are really hard to hit in the JIT right now, because
guard-relaxation likes to relax away a lot of specialized object
guards, and cases where the result of instanceof is known statically are
removed by some combination of hphpc and hhbbc.

That said, I would still like to be able to simplify them in the JIT.

I tested this by turning off guard-relaxation and looking at some
examples, plus running the test suite.

Reviewed By: @swtaarrs

Differential Revision: D1738285

Signature: t1:1738285:1418721826:6622d0c7da0ba7a63d317d28ad6c744eb3e65248