Revert "[X86][ABI] Don't preserve return regs for preserve_all/preserve_most CCs"

This caused Chromium to crash, see comment on the code review.

> Currently both calling conventions preserve registers that are used to
> store a return value. This causes the returned value to be lost:
>
>   define i32 @bar() {
>     %1 = call preserve_mostcc i32 @foo()
>     ret i32 %1
>   }
>
>   define preserve_mostcc i32 @foo() {
>     ret i32 2
>     ; preserve_mostcc will restore %rax,
>     ; whatever it was before the call.
>   }
>
> This contradicts the current documentation (preserve_allcc "behaves
> identical to the `C` calling conventions on how arguments and return
> values are passed") and also breaks [[clang::preserve_most]].
>
> This change makes CSRs be preserved iff they are not used to store a
> return value (e.g. %rax for scalars, {%rax:%rdx} for __int128, %xmm0
> for double). For void functions no additional registers are
> preserved, i.e. the behaviour is backward compatible with existing
> code.
>
> Differential Revision: https://reviews.llvm.org/D141020

This reverts commit 0276fa89.