Comment 1 for bug 839001

Gcc seems to be generating a 4-byte wide strlen scanner during -O3 which is tripping up valgrind. I'm not sure if this should be considered a gcc bug or a valgrind bug:

-O1:
  4005a2: b8 00 00 00 00 mov $0x0,%eax
  4005a7: 48 c7 c1 ff ff ff ff mov $0xffffffffffffffff,%rcx
  4005ae: f2 ae repnz scas %es:(%rdi),%al
  4005b0: 48 f7 d1 not %rcx

-O3:
  4004b8: 48 89 c6 mov %rax,%rsi
  4004bb: 8b 0e mov (%rsi),%ecx
  4004bd: 48 83 c6 04 add $0x4,%rsi
  4004c1: 8d 91 ff fe fe fe lea -0x1010101(%rcx),%edx
  4004c7: f7 d1 not %ecx
  4004c9: 21 ca and %ecx,%edx
  4004cb: 81 e2 80 80 80 80 and $0x80808080,%edx
  4004d1: 74 e8 je 4004bb <main+0x1b>
  4004d3: 89 d1 mov %edx,%ecx
  4004d5: 48 89 c7 mov %rax,%rdi
  4004d8: c1 e9 10 shr $0x10,%ecx
  4004db: f7 c2 80 80 00 00 test $0x8080,%edx
  4004e1: 0f 44 d1 cmove %ecx,%edx
  4004e4: 48 8d 4e 02 lea 0x2(%rsi),%rcx
  4004e8: 48 0f 44 f1 cmove %rcx,%rsi
  4004ec: 00 d2 add %dl,%dl
  4004ee: 48 83 de 03 sbb $0x3,%rsi
  4004f2: 48 29 c6 sub %rax,%rsi