ExecutionEngine.cpp: Move execution engine creation stuff into a new

  static method here.
 Remove some extra blank lines.
ExecutionEngine.h: Add its prototype.
lli.cpp: Call it.

Make creation method for each type of EE into a static method of its
own subclass.

Interpreter/Interpreter.cpp: ExecutionEngine::createInterpreter -->
 Interpreter::create
Interpreter/Interpreter.h: Likewise.
JIT/JIT.cpp: ExecutionEngine::createJIT --> VM::create
JIT/VM.h: Likewise.

llvm-svn: 8343