Fix back-branch target for do-while loops.

To ensure back branches always go to a header block, create a header
block even for !testFirst loops.  Then unify common code between the
testFirst/!testFirst cases.

Generate the header-block code first, so update golden files.

Realize that certain infinite loops generate invalid SPIR-V, so put a
TODO to instead abort code generation in such cases.

Change-Id: I1e173c8f73daad186cfc666b7d72bd563ed7665d

Test/spv.for-notest.vert

@@ -2,5 +2,8 @@
 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; }
 }