As discussed on IRC, I reduced the tests to a very bare minimum. The previous 
tests will come back very soon with a second patch. Compared to the previous 
patch, I fixed the last __is_narrowing_conversion bug for real.

New patch is attached. Or view it on GitHub:
https://github.com/gcc-mirror/gcc/compare/master...mattkretz:2cc3bb9d9

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 Dr. Matthias Kretz                           https://mattkretz.github.io
 GSI Helmholtz Centre for Heavy Ion Research               https://gsi.de
 std::experimental::simd              https://github.com/VcDevel/std-simd
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