Auto merge of #106967 - saethlin:remove-vec-as-ptr-assume, r=thomcc

Remove the assume(!is_null) from Vec::as_ptr

At a guess, this code is leftover from LLVM was worse at keeping track of the niche information here. In any case, we don't need this anymore: Removing this `assume` doesn't get rid of the `nonnull` attribute on the return type.

Author: bors