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

SPIRV/SpvBuilder.h

@@ -374,7 +374,7 @@ public:
     void endSwitch(std::vector<Block*>& segmentBB);
 
     struct LoopBlocks {
-        Block &body, &merge, &continue_target;
+        Block &head, &body, &merge, &continue_target;
     };
 
     // Start a new loop and prepare the builder to generate code for it.  Until