[FastISel] Correctly report prototype on miss

[aengelke]

Summary

The type of a function is nowadays just an opaque pointer, which is not
helpful when analyzing FastISel misses. Instead print the actual
function type of the function.

[efriedma]

LGTM

llvm/lib/CodeGen/SelectionDAG/SelectionDAGISel.cpp

if (!FastIS->lowerArguments()) {
       FastISelFailed = true;
       // Fast isel failed to lower these arguments
       ++NumFastIselFailLowerArguments;
 
       OptimizationRemarkMissed R("sdagisel", "FastISelFailure",
                                  Fn.getSubprogram(),
                                  &Fn.getEntryBlock());
       R << "FastISel didn't lower all arguments: "
-        << ore::NV("Prototype", Fn.getType());
+        << ore::NV("Prototype", Fn.getFunctionType());
       reportFastISelFailure(*MF, *ORE, R, EnableFastISelAbort > 1);
 
       // Use SelectionDAG argument lowering
       LowerArguments(Fn);
       CurDAG->setRoot(SDB->getControlRoot());
       SDB->clear();
       CodeGenAndEmitDAG();
     }