A better API to evaluate or precompute an expression

[dcodeIO]

We are currently utilizing precompute to find out whether conditions to branches are constant by

• creating a temporary function
• using the expression of interest as its body
• running the precompute pass on that function
• evaluating the new body
• removing the temporary function again.

This has downsides in regards to overhead and that we have to replace the expression instead of keeping it intact, leading to not-so-untouched output, and doesn't work if the expression has side-effects.

So I've been thinking about adding an API like BinaryenExpressionEvaluate to aid here, but since precompute is a pass with no header that isn't meant to be used this way, it seems to require substantial code changes that I'm unsure about. Any suggestions how to tackle this? :)

[tlively]

This shouldn't be too difficult because the interpretation machinery in wasm-interpreter.h is already independent of any particular pass. I would look at how Precompute.cpp integrates the interpreter and do something similar in one of the utils headers, rather than trying to reuse Precompute itself.

[tlively]

Looks like this was done in #2702.