Make exprIsConApp_maybe a bit cleverer

Addresses #23159.

See Note Note [Exploit occ-info in exprIsConApp_maybe]
in GHC.Core.SimpleOpt.

Compile times go down very slightly, but always go down,
never up.  Good!

Metrics: compile_time/bytes allocated
------------------------------------------------
 CoOpt_Singletons(normal)   -1.8%
           T15703(normal)   -1.2% GOOD

                geo. mean   -0.1%
                minimum     -1.8%
                maximum     +0.0%

Metric Decrease:
    CoOpt_Singletons
    T15703