[Bug tree-optimization/64622] convoluted loop codegen for __strcspn_c1

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

https://gcc.gnu.org/bugzilla/show_bug.cgi?id=64622

--- Comment #3 from Richard Biener <rguenth at gcc dot gnu.org> ---
without loop header copyign we generate

__strcspn_c1:
.LFB0:
        .cfi_startproc
        xorl    %eax, %eax
        jmp     .L2
        .p2align 4,,10
        .p2align 3
.L8:
        cmpl    %esi, %edx
        je      .L6
        addq    $1, %rax
.L2:
        movsbl  (%rdi,%rax), %edx
        testb   %dl, %dl
        jne     .L8
.L6:
        rep ret

so it would be interesting to investigate how they do this (if it's a special
hack or some systematic fix).  The loop header contains just the IV increment
here.