Blimey!  CoreUtils.exprIsHNFlike had not one but two bugs.

 * is_hnf_like treated coercion args like type args
   (result: exprIsHNF might wrongly say True)

 * app_is_value treated type args like value args
   (result: exprIsHNF might wrongly say False)

Bizarre.  This goes back to at least 2012. It's amazing that it
hasn't caused more trouble.

It was discovered by a Lint error when compiling Trac #11248 with -O.