[X86] Always return the sret parameter in eax/rax, even on 32-bit

Summary:
This rule was always in the old SysV i386 ABI docs and the new ones that
H.J. Lu has put together, but we never noticed:

  EAX   scratch register; also used to return integer and pointer values
        from functions; also stores the address of a returned struct or union

Fixes PR23491.

Reviewers: majnemer

Subscribers: llvm-commits

Differential Revision: http://reviews.llvm.org/D9715

llvm-svn: 237175