Rollup merge of #56789 - alexcrichton:simd_select_bitmask, r=rkruppe

rustc: Add an unstable `simd_select_bitmask` intrinsic

This is going to be required for binding a number of AVX-512 intrinsics
in the `stdsimd` repository, and this intrinsic is the same as
`simd_select` except that it takes a bitmask as the first argument
instead of a SIMD vector. This bitmask is then transmuted into a `<NN x
i8>` argument, depending on how many bits it is.

cc rust-lang-nursery/stdsimd#310