Try using MCoercion in exprIsConApp_maybe

This is just a simple refactor that makes exprIsConApp_maybe
a little bit more direct, simple, and efficient.

Metrics: compile_time/bytes allocated
    geo. mean                                          -0.1%
    minimum                                            -2.0%
    maximum                                            -0.0%

Not a big gain, but worthwhile given that the code is, if anything,
easier to grok.