simd, redundant pcmpeqb and pxor

[i.nixman@autistici.org]

Hello,

look at this example(https://godbolt.org/z/TnGMsfMs6):
```
auto foo(const char *p) {
     const auto substr = _mm_loadu_si128((const __m128i *)p);
     return _mm_cmplt_epi8(substr, _mm_set1_epi8('0'));
}
```
and to the generated asm:
```
1: foo(char const*):
2:    movdqu  xmm0, XMMWORD PTR [rdi]
3:    pxor    xmm1, xmm1
4:    pcmpgtb xmm0, XMMWORD PTR .LC0[rip]
5:    pcmpeqb xmm0, xmm1
6:    ret
```
look at line 5.
is there any reason for `pcmpeqb` instruction?

just for info, clang's output(https://godbolt.org/z/MPnvEMdhr):
```
1: foo(char const*):
2:    movdqu  xmm1, xmmword ptr [rdi]
3:    movdqa  xmm0, xmmword ptr [rip + .LCPI0_0]
4:    pcmpgtb xmm0, xmm1
5:    ret
```


best!