[dfmc,dfm output] Clearly indicate CONGRUENT-CALLs.

In practice, this changes CALLi to CONGRUENT-CALLi.

This helps make it clear that a congruent call is happening rather
than a fully generic dispatch (CALLg, etc). This also makes it match
the terminology used in the run-times and generated code more
closely.
Merge pull request #759 from waywardmonkeys/debug-back-end-congruent-call

[dfmc,dfm output] Clearly indicate CONGRUENT-CALLs.