add missing ^^ and ^^= arrayop support

[MartinNowak]

fixup for #1891, required by dlang/dmd#7032.
Unfortunately not really possible to test those with druntime's unittests as PowExp depends on phobos' std.math.pow.

[dlang-bot]

Thanks for your pull request, @MartinNowak!

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[ibuclaw]

It may not be a bad idea to move it to druntime. As phobos should be optional.

[MartinNowak]

It may not be a bad idea to move it to druntime. As phobos should be optional.

Yes indeed, let's take this as a hint that having the compiler rely on phobos functions as intrinsics is not the best idea. In fact gcc/gdc's explicit low-level intrinsics seem like a much better approach, easy enough to wrap those where wanted now that inlining works properly.