[Bug target/79709] Subobtimal code with -mavx and explicit vector

["tkoenig at gcc dot gnu.org" <gcc-bugzilla at gcc dot gnu dot org>]

https://gcc.gnu.org/bugzilla/show_bug.cgi?id=79709

--- Comment #9 from Thomas Koenig <tkoenig at gcc dot gnu.org> ---
The generated code for the loop seems to be on par with what
clang and icc do, so that part is fixed.

Initialization is strange for icc. For clang, it really quite short:

foo:                                    # @foo
        .cfi_startproc
# BB#0:
        vxorps  %ymm2, %ymm2, %ymm2
        vmovapd .LCPI0_0(%rip), %ymm8   # ymm8 =
[4.000000e+00,4.000000e+00,4.000000e+00,4.000000e+00]
        vmovapd %ymm1, %ymm4
        vmovapd %ymm0, %ymm5
        .p2align        4, 0x90
.LBB0_1:                                # =>This I

vs. gcc:

foo:
.LFB0:
        .cfi_startproc
        vmovsd  .LC0(%rip), %xmm2
        vmovapd %ymm1, %ymm7
        vpxor   %xmm5, %xmm5, %xmm5
        vmovq   %xmm2, %xmm9
        vmulpd  %ymm1, %ymm1, %ymm10
        vmovapd %xmm9, %xmm9
        vunpcklpd       %xmm2, %xmm9, %xmm3
        vinsertf128     $0x0, %xmm3, %ymm9, %ymm9
        vextractf128    $0x1, %ymm9, %xmm3
        vmovsd  %xmm2, %xmm3, %xmm3
        vinsertf128     $0x1, %xmm3, %ymm9, %ymm9
        vextractf128    $0x1, %ymm9, %xmm3
        vunpcklpd       %xmm2, %xmm3, %xmm3
        vmovsd  .LC1(%rip), %xmm2
        vmovq   %xmm2, %xmm8
        vinsertf128     $0x1, %xmm3, %ymm9, %ymm9
        vmovapd %xmm8, %xmm8
        vunpcklpd       %xmm2, %xmm8, %xmm3
        vinsertf128     $0x0, %xmm3, %ymm8, %ymm8
        vextractf128    $0x1, %ymm8, %xmm3
        vmovsd  %xmm2, %xmm3, %xmm3
        vinsertf128     $0x1, %xmm3, %ymm8, %ymm8
        vextractf128    $0x1, %ymm8, %xmm3
        vunpcklpd       %xmm2, %xmm3, %xmm3
        vinsertf128     $0x1, %xmm3, %ymm8, %ymm8
        vmovapd %ymm0, %ymm3
        jmp     .L3
        .p2align 4,,10
        .p2align 3
.L13: