fix #22281 [REG master] Wrong associative array access with alias this for key

[rainers]

don't omit the cast in aa[cast(K)key] if key converts via alias this, so that the proper type is inferred by _d_aaGetY

[dlang-bot]

Thanks for your pull request, @rainers!

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dub run digger -- build "master + dmd#22283"