Vyper's `extract32` can ready dirty memory

Vyper's `extract32` can ready dirty memory. When using the built-in `extract32(b, start)`, if the `start` index provided has for side effect to update `b`, dirty memory may be read and returned by `extract32`. Before evaluating `start`, the function `Extract32.build_IR` caches only the pointer in memory/storage and the length of `b`, but not the actual content of `b`, leading to outdated length being used with new content when extracting 32 bytes from `b`. A PoC in a contract shows that `extract32` accessed dirty memory, impacting affected contracts by returning unexpected output.