[AVX] If the data which is going to be saved is already in two XMM registers

(for example, after integer operation), do not pack the registers into a YMM
before saving. Its better to save as two XMM registers.

Before:
                vinsertf128         $1, %xmm3, %ymm0, %ymm3
                vinsertf128         $0, %xmm1, %ymm3, %ymm1
                vmovaps              %ymm1, 416(%rsp)

After:
                vmovaps              %xmm3, 416+16(%rsp)
                vmovaps              %xmm1, 416(%rsp)

llvm-svn: 137308