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