Major refactoring of the bytecode reader. This includes the following

changes:
  * BytecodeReader::getType(...) used to return a null pointer
    on error.  This was only checked about half the time.  Now we convert
    it to throw an exception, and delete the half that checked for error.
    This was checked in before, but psmith crashed and lost the change :(
  * insertValue no longer returns -1 on error, so callers don't need to
    check for it.
  * Substantial rewrite of InstructionReader.cpp, to use more efficient,
    simpler, data structures.  This provides another 5% speedup.  This also
    makes the code much easier to read and understand.

llvm-svn: 8984