Revert r363116 "[X86] [ABI] Fix i386 ABI "__m64" type bug"

This introduced MMX instructions in code that wasn't previously using
them, breaking programs using 64-bit vectors and x87 floating-point in
the same application. See discussion on the code review for more
details.

> According to System V i386 ABI: the  __m64 type paramater and return
> value are passed by MMX registers. But current implementation treats
> __m64 as i64 which results in parameter passing by stack and returning
> by EDX and EAX.
>
> This patch fixes the bug (https://bugs.llvm.org/show_bug.cgi?id=41029)
> for Linux and NetBSD.
>
> Patch by Wei Xiao (wxiao3)
>
> Differential Revision: https://reviews.llvm.org/D59744

llvm-svn: 363790