[TableGen] FieldInit: improve assertion message

[lebedev.ri]

Summary

When fiddling with sched profiles, especially creating new ones, it's amazingly easy
to end up with malformed .td that crashes tablegen, without explanation of the bug.
This changes the most common assertion i have encountered to dump enough information
to be able to fix the .td

Split of from D63628

[lebedev.ri]

To be noted this is essentially NFC, this only improves the error reporting, it does not change any error detection logic.

[craig.topper]

LGTM

llvm/trunk/include/llvm/TableGen/Record.h

 
 /// X.Y - Represent a reference to a subfield of a variable
 class FieldInit : public TypedInit {
   Init *Rec;                // Record we are referring to
   StringInit *FieldName;    // Field we are accessing
 
   FieldInit(Init *R, StringInit *FN)
       : TypedInit(IK_FieldInit, R->getFieldType(FN)), Rec(R), FieldName(FN) {
-    assert(getType() && "FieldInit with non-record type!");
+#ifndef NDEBUG
+    if (!getType()) {
+      llvm::errs() << "In Record = " << Rec->getAsString()
+                   << ", got FieldName = " << *FieldName
+                   << " with non-record type!\n";
+      llvm_unreachable("FieldInit with non-record type!");
+    }
+#endif
   }
 
 public:
   FieldInit(const FieldInit &) = delete;
   FieldInit &operator=(const FieldInit &) = delete;
 
   static bool classof(const Init *I) {
     return I->getKind() == IK_FieldInit;