Split loop header from condition testing for for/while loops.

Test/spv.for-notest.vert

@@ -2,8 +2,5 @@
 layout(location=0) out highp int r;
 void main() {
   int i;
-  // This infinite loop results in bad SPIR-V generated, since the merge block
-  // is dropped as unreachable.  It is still useful for testing the rest of the
-  // code generation.
   for (i=0; ; i++) { r = i; }
 }