Fix zero-length struct/union test. Remove nonsensical test.

The comment suggests this was meant to detect unions, but in fact it
compared f->c, the union/struct size, against f->next->c, the first
element's offset.

This affected only zero-length structs/unions with a first (zero-length)
element, as in this code:

    struct u2 {
    };

    struct u {
      struct u2 u2;
    } u;

    struct u f(struct u x)
    {
      return x;
    }

However, such structures turned out to be broken anyway, as code like this
was generated for the above f:

0000000000000000 <f>:
   0:   55                      push   %rbp
   1:   48 89 e5                mov    %rsp,%rbp
   4:   48 81 ec 10 00 00 00    sub    $0x10,%rsp
   b:   66 0f d6 45 f8          movq   %xmm0,-0x8(%rbp)
  10:   66 0f 6e 45 f8          movd   -0x8(%rbp),%xmm0
  15:   e9 00 00 00 00          jmpq   1a <f+0x1a>
  1a:   c9                      leaveq
  1b:   c3                      retq

1 files changed, 1 insertions, 5 deletions

diff --git a/x86_64-gen.c b/x86_64-gen.c
index d837a1d..90c8247 100644
--- a/x86_64-gen.c
+++ b/x86_64-gen.c
@@ -1066,12 +1066,8 @@ static X86_64_Mode classify_x86_64_inner(CType *ty)
     case VT_STRUCT:
         f = ty->ref;
 
-        // Detect union
-        if (f->next && (f->c == f->next->c))
-          return x86_64_mode_memory;
-        
         mode = x86_64_mode_none;
-        for (; f; f = f->next)
+        for (f = f->next; f; f = f->next)
             mode = classify_x86_64_merge(mode, classify_x86_64_inner(&f->type));
         
         return mode;