Auto merge of #121225 - RalfJung:simd-extract-insert-const-idx, r=oli-obk,Amanieu

require simd_insert, simd_extract indices to be constants

As discussed in rust-lang/rust#77477 (see in particular [here](rust-lang/rust#77477 (comment))). 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 rust-lang/stdarch#1530 propagating to the rustc repo.

Author: bors