Default-constructed Pass/PassManager will have a MessageConsumer
which ignores all messages. SetMessageConsumer() should be called
to supply a meaningful MessageConsumer.
Also removed the default argument value of `skip_nop` for function
`SinglePassRunAndCheck()` and `SinglePassRunAndDisassemble()`. This is
required to support variadic arguments.
Previously we use vectors of objects and move semantics to handle
ownership. That approach has the flaw that inserting an object into
the middle of a vector, which may trigger a vector reallocation,
can invalidate some addresses taken from instructions.
Now the in-memory representation internally uses vector of unique
pointers to handle ownership. Since objects are explicitly heap-
allocated now, pointers to them won't be invalidated by vector
resizing anymore.
Add a pass to freeze spec constants to their default values. This pass does
not fold the frozen spec constants and does not handle SpecConstantOp
instructions and SpecConstantComposite instructions.
