bad codegen on structured gather

[mattkretz]

Vc version / revision	Operating System	Compiler & Version	Compiler Flags	Assembler & Version	CPU
0a7836f	Linux	GCC 8	-DNDEBUG -std=c++17 -O2 -march=skylake

Testcase

struct X { float a; int b; };

int_v gather(X *ptr, int_v idx) {
    return Vc::span<X>(ptr, 100)[idx][&X::b];
}

Actual Results

gather(X*, Vc_1::Vector<int, Vc_1::VectorAbi::Avx>):
  pushq %rbp
  addq $4, %rdi
  movq %rsp, %rbp
  andq $-32, %rsp
  subq $40, %rsp
  leaq -120(%rsp), %rax
  leaq 32(%rax), %rdx
  vmovdqa %ymm0, -88(%rsp)
  vmovdqa %ymm0, -24(%rsp)
  vmovdqa %ymm0, -120(%rsp)
.L2:
  sall (%rax)
  addq $4, %rax
  cmpq %rdx, %rax
  jne .L2
  vmovdqa -120(%rsp), %ymm2
  vpcmpeqd %ymm1, %ymm1, %ymm1
  vpgatherdd %ymm1, (%rdi,%ymm2,4), %ymm0
  leave
  ret

Expected Results

  vpcmpeqd %ymm1, %ymm1, %ymm1
  vpgatherdd %ymm1, (%rdi,%ymm0,8), %ymm2
  vmovdqa %ymm2, %ymm0
  ret