Add {gather,scatter}_select_unchecked
Add {gather,scatter}_select_unchecked

This unsafe variant allows the thinnest API, in case LLVM cannot
perform loop-invariant code motion on a hot loop when the safe
form is used.

An unchecked variant could be added to other forms, but doesn't
seem likely to improve anything, since it would just add heavier
codegen.

aidancully/rust,aidancully/rust,graydon/rust,graydon/rust,graydon/rust,aidancully/rust,graydon/rust,graydon/rust,aidancully/rust,graydon/rust,aidancully/rust,aidancully/rust