1741396 - Use dynamic_cast in debug-only MDefinition::to* when --enable-cpp-rtti is enabled

Status: NEW

(Core :: JavaScript Engine: JIT, enhancement, P3)

Description

[Julian Seward [:jseward]]

MDefinition::to* do downcasts in the MIR node hierarchy using static_cast,
the correctness of which depends on the target node's op field being set correctly.
Setting it incorrectly makes it possible to use MDefinition::to* to cast invalidly,
which is potentially dangerous, particularly given that many nodes have similar
layouts.

In debug builds, when configured with --enable-cpp-rtti, it would be nice to have
those be dynamic_cast instead, so that any such mistakes will get caught at runtime.
This has proven to be helpful for example for the development in bug 1713092.