Auto merge of #57269 - gnzlbg:simd_bitmask, r=rkruppe

Add intrinsic to create an integer bitmask from a vector mask

This PR adds a new simd intrinsic: `simd_bitmask(vector) -> unsigned integer` that creates an integer bitmask from a vector mask by extracting one bit of each vector lane.

This is required to implement: https://github.com/rust-lang-nursery/packed_simd/issues/166 .

EDIT: the reason we need an intrinsics for this is that we have to truncate the vector lanes to an `<i1 x N>` vector, and then bitcast that to an `iN` integer (while making sure that we only materialize `i8`, ... , `i64` - that is, no `i1`, `i2`, `i4`, types), and we can't do any of that in a Rust library.

r? @rkruppe

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