require simd_insert, simd_extract indices to be constants As discussed in https://github.com/rust-lang/rust/issues/77477 (see in particular [here](https://github.com/rust-lang/rust/issues/77477#issuecomment-703149102)). This PR doesn't touch codegen yet -- the first step is to ensure that the indices are always constants; the second step is to then make use of this fact in backends. Blocked on https://github.com/rust-lang/stdarch/pull/1530 propagating to the rustc repo.