Reland "[X86][ABI] Don't preserve return regs for preserve_all/preserve_most CCs""
The original change mistakenly excluded parameter registers from the
list of callee-saved-registers. This reland fixes it - it only excludes
the return registers for preserve_all/preserve_most CCs.
Original description:
> 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/D143425