Revert "Move "auto-init" instructions to the dominator of their users"

This could also move initialization of sret args, causing actually
initialized parts of such return values to be uninitialized. See
discussion on the code review.

> As a result of -ftrivial-auto-var-init, clang generates instructions to
> set alloca'd memory to a given pattern, right after the allocation site.
> In some cases, this (somehow costly) operation could be delayed, leading
> to conditional execution in some cases.
>
> This is not an uncommon situation: it happens ~500 times on the cPython
> code base, and much more on the LLVM codebase. The benefit greatly
> varies on the execution path, but it should not regress on performance.
>
> This is a recommit of cca01008 with
> MemorySSA update fixes.
>
> Differential Revision: https://reviews.llvm.org/D137707

This reverts commit 50b2a113
and follow-up commit ad9ad373.