cmd/compile: omit racefuncentry/exit when they are not needed

[dvyukov]

We only need racefuncentry/exit if the function also contains instrumented memory accesses and/or other calls (racefuncentry/exit are used to restore stack traces, if there is nothing that will require stack traces, then no point in these calls as well).
This was done in LLVM from day one:
https://github.com/llvm-mirror/llvm/blob/master/lib/Transforms/Instrumentation/ThreadSanitizer.cpp#L471
and I was under impression that we are doing this in gc too. But it seems that we don't. A good example is internal/bytealg.countGenericString:

0000000000425420 <internal/bytealg.countGenericString>:
  425420:       64 48 8b 0c 25 f8 ff    mov    %fs:0xfffffffffffffff8,%rcx
  425427:       ff ff 
  425429:       48 3b 61 10             cmp    0x10(%rcx),%rsp
  42542d:       76 6b                   jbe    42549a <internal/bytealg.countGenericString+0x7a>
  42542f:       48 83 ec 18             sub    $0x18,%rsp
  425433:       48 89 6c 24 10          mov    %rbp,0x10(%rsp)
  425438:       48 8d 6c 24 10          lea    0x10(%rsp),%rbp
  42543d:       48 8b 44 24 18          mov    0x18(%rsp),%rax
  425442:       48 89 04 24             mov    %rax,(%rsp)
  425446:       e8 d5 85 05 00          callq  47da20 <runtime.racefuncenter>
  42544b:       48 8b 44 24 28          mov    0x28(%rsp),%rax
  425450:       0f b6 4c 24 30          movzbl 0x30(%rsp),%ecx
  425455:       48 8b 54 24 20          mov    0x20(%rsp),%rdx
  42545a:       31 db                   xor    %ebx,%ebx
  42545c:       31 f6                   xor    %esi,%esi
  42545e:       eb 17                   jmp    425477 <internal/bytealg.countGenericString+0x57>
  425460:       48 8d 7b 01             lea    0x1(%rbx),%rdi
  425464:       44 0f b6 04 1a          movzbl (%rdx,%rbx,1),%r8d
  425469:       4c 8d 4e 01             lea    0x1(%rsi),%r9
  42546d:       41 38 c8                cmp    %cl,%r8b
  425470:       49 0f 44 f1             cmove  %r9,%rsi
  425474:       48 89 fb                mov    %rdi,%rbx
  425477:       48 39 c3                cmp    %rax,%rbx
  42547a:       7c e4                   jl     425460 <internal/bytealg.countGenericString+0x40>
  42547c:       48 89 74 24 08          mov    %rsi,0x8(%rsp)
  425481:       e8 da 85 05 00          callq  47da60 <runtime.racefuncexit>
  425486:       48 8b 44 24 08          mov    0x8(%rsp),%rax
  42548b:       48 89 44 24 38          mov    %rax,0x38(%rsp)
  425490:       48 8b 6c 24 10          mov    0x10(%rsp),%rbp
  425495:       48 83 c4 18             add    $0x18,%rsp
  425499:       c3                      retq   
  42549a:       e8 51 55 05 00          callq  47a9f0 <runtime.morestack_noctxt>
  42549f:       e9 7c ff ff ff          jmpq   425420 <internal/bytealg.countGenericString>

@mdempsky @josharian

[gopherbot]

Change https://golang.org/cl/121235 mentions this issue: cmd/compile: omit racefuncentry/exit when they are not needed