Optimizations for power operator a^b

[MikePall]

The current narrowing and strength reduction for a^b aka math.pow(a, b) is problematic.

The a^i integer narrowing is definitely useful:

• It opens other optimization opportunities (unrolling).
• It gives more precise results than the exp/log method in some important cases, like 3^i.
• But it may give different results than calling the libm pow() function. Whichever is more accurate is a question of the libm implementation and some debate. Especially when some intermediate step and/or the final result overflows FP precision.

The split into exp2(b*log2(a)) is of questionable utility and more troublesome:

• It mainly benefits some silly benchmarks, where log2(a) can be hoisted out.
• It doesn't give proper results for NaN, Inf and other special cases. Whether the libm implementation does that is often questionable, though.
• It adds a dependency on C99 exp2() and log2(), which (still) aren't part of some libm implementations.
• And/or their and our exp2() and log2() workaround implementations suck: exp(x_k) and log(x_k) are hopelessly imprecise.
• All JIT backends have to do some weird dance to join these back together to pow() in the hope that this fixes some of these issues.

Definitely needs further discussion and analysis.

[MikePall]

Done.