`sandboxir::Context` is defined at a pass-level scope with the
`SandboxVectorizerPass` class because the function pass manager `FPM`
object depends on it, and that is in pass-level scope to avoid
recreating the pass pipeline every single time `runOnFunction()` is
called.
This means that the Context's state lives on across function passes. The
problem is twofold:
(i) the LLVM IR to Sandbox IR map can grow very large including objects
from different functions, which is of no use to the vectorizer, as it's
a function-level pass.
(ii) this can result in stale data in the LLVM IR to Sandbox IR object
map, as other passes may delete LLVM IR objects.
To fix both issues this patch introduces a `Context::clear()` function
that clears the `LLVMValueToValueMap`.
This patch removes the assertion that checks for an existing function.
If one exists it will remove it and create a new one. This helps remove
a crash when a function declaration object already exists and we are
about to create a SandboxIR object for the definition.
The `EraseCallbackID` field is not always initialized in the ctor for
SeedCollector; if not, it will be used uninitialized by its dtor. This
could potentially lead to the erasure of a random callback, leading to a
bug.
Fixed by making `CallbackID` an opaque type, which is always
default-initialized to an invalid ID.
