Rollup merge of #41206 - eddyb:avoid-illegal-vectors, r=nagisa

Fix pairs of doubles using an illegal <8 x i8> vector.

Accidentally introduced in #40658 and discovered in some Objective-C bindings (returning `NSPoint`).
Turns out LLVM will widen element types of illegal vectors instead of increasing element count, i.e. it will zero-extend `<8 x i8>` to `<8 x i16>`, interleaving the bytes, instead of using the first 8 of `<16 x i8>`.