Workaround Issue 11497 - lambda in "static if"/"assert" prevent inlining of functio

This little "trivial" change is all that is required for `emplace` to be always inlined at call site (regardless of `-inline` switch) with DMD.

This gives *drastic* performance improvements for the likes of `array`, `Appender`, `Array`.

I had to use `lvalueOf!Arg` instead of `Arg.init`, because the point of the test is to check for postblit, and `Arg.init` specifically bypasses it. I left in the old code, so we can revert if/when 11497 is fixed.