Precompute power if both arguments are constants

[MaxGraey]

@dcodeIO I know we already talked about this and I understand this not general solution but until we haven't own IR I propose do this due to expessions like 10 ** 5, 2 ** 23 and etc is pretty popular as I see.

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[dcodeIO]

What do you think of making this a new pass in PostAssemblyScript.cpp on the Binaryen end? Would imagine that we can do much more there, in this case for example postprocessing calls to ~lib/math/ipow32. Also doesn't require doing additional work in untouched output, so creating a debug build is always fast.

[MaxGraey]

Yeah, it could be binaryen's pass. But this PR useful in any case even with our custom Binaryen pass due to it works for untouched version as well. Ofc for simple cases

[MaxGraey]

Btw we could extend values like 2 ** 40 and other exp >= 32 to i64 during type deducing instead stay with overflowed i32. wdyt?