[GVN] hasUsersIn - merge isa<> and cast<> into single dyn_cast<> and convert...

[GVN] hasUsersIn - merge isa<> and cast<> into single dyn_cast<> and convert for-range loop to any_of() test. NFCI.

Avoid running isa<> and cast<> if we can - dyn_cast<> can more efficiently check for a safe cast and give the casted pointer.