Add LLVM patch D14260 to improve SIMD code

[eschnett]

Arch Robison proposed the patch http://reviews.llvm.org/D14260 "Optimize store of "bitcast" from vector to aggregate" for LLVM. This patch applies cleanly to LLVM 3.7.1. It seems to be the last missing puzzle piece on the LLVM side to allow generating efficient SIMD instructions via llvm_call in Julia. For an example package, see e.g. https://github.com/eschnett/SIMD.jl.

Some discussion relevant to this PR are in #5355. @ArchRobison, please comment.

Julia stores tuples as LLVM arrays, whereas LLVM SIMD instructions require LLVM vectors. The respective conversions are unfortunately not always optimized out unless the patch above is applied, leading to a cumbersome sequence of instructions to disassemble and reassemble a SIMD vector. An example is given here eschnett/SIMD.jl#1 (comment).

Without this patch, the loop kernel looks like (x86-64, AVX2 instructions):

    vunpcklpd   %xmm4, %xmm3, %xmm3 # xmm3 = xmm3[0],xmm4[0]
    vunpcklpd   %xmm2, %xmm1, %xmm1 # xmm1 = xmm1[0],xmm2[0]
    vinsertf128 $1, %xmm3, %ymm1, %ymm1
    vmovupd 8(%rcx), %xmm2
    vinsertf128 $1, 24(%rcx), %ymm2, %ymm2
    vaddpd  %ymm2, %ymm1, %ymm1
    vpermilpd   $1, %xmm1, %xmm2 # xmm2 = xmm1[1,0]
    vextractf128    $1, %ymm1, %xmm3
    vpermilpd   $1, %xmm3, %xmm4 # xmm4 = xmm3[1,0]
Source line: 62
    vaddsd  (%rcx), %xmm0, %xmm0

Note that the SIMD vector is kept in register %ymm1, but is unnecessarily scalarized into registers %xmm{0,1,2,3} at the end of the kernel, and re-assembled in the beginning.

With this patch, the loop kernel looks like:

L192:
    vaddpd  (%rdx), %ymm1, %ymm1
Source line: 62
    addq    %rsi, %rdx
    addq    %rcx, %rdi
    jne L192

which is perfect.

[tkelman]

why not just adjust the patch content so it can be applied consistently with all the others instead of duplicating application code

[eschnett]

I wanted to keep the original LLVM patch without modification.

I can, of course, modify it -- but then it might be easier to just append it to one of the existing patches instead of introducing a new one.

[tkelman]

Modifying the existing patches would lead to the .patch-applied files indicating an incorrect state in everyone's existing build, so the new modifications would not get applied except in from-scratch builds of llvm.

[eschnett]

Addressed @tkelman's comments, rebased.