Truthful error reporting for complicated compile-time type mismatches

Truthful error reporting for complicated compile-time type mismatches

Type mismatches for multiple-use LVARs (i.e. resulting from conditional
expressions) can't be pinpointed to a single source for the value(s).

Such expressions used to be reported as type mismatches with the constant
NIL. Instead, switch to a more complex format with the lowest common source
form, if any (hopefully the conditional), and the nodes that may deliver
the form's value.

Do the same when warning about non-EQ-comparable CATCH tags.