TableGen: Support folding casts from bits to int

[arsenm]

Summary

This is to fix an incorrect error when trying to initialize
DwarfNumbers with a !cast<int> of a bits initializer.
getValuesAsListOfInts("DwarfNumbers") would not see an IntInit
and instead the cast, so would give up.

It seems likely that this could be generalized to attempt
the convertInitializerTo for any type. I'm not really sure
why the existing code seems to special case the string cast cases
when convertInitializerTo seems like it should generally handle this
sort of thing.

[arsenm]

r243722

lib/TableGen/Record.cpp

if (isa<StringRecTy>(getType())) {
         }
         assert(CurRec && "NULL pointer");
         if (Record *D = (CurRec->getRecords()).getDef(Name))
           return DefInit::get(D);
 
         PrintFatalError(CurRec->getLoc(),
                         "Undefined reference:'" + Name + "'\n");
       }
+
+      if (isa<IntRecTy>(getType())) {
+        if (BitsInit *BI = dyn_cast<BitsInit>(LHS)) {
+          if (Init *NewInit = BI->convertInitializerTo(IntRecTy::get()))
+            return NewInit;
+          break;
+        }
+      }
     }
     break;
   }
   case HEAD: {
     if (ListInit *LHSl = dyn_cast<ListInit>(LHS)) {
       assert(!LHSl->empty() && "Empty list in head");
       return LHSl->getElement(0);
     }

test/TableGen/cast-list-initializer.td

+// RUN: llvm-tblgen %s | FileCheck %s
+
+class Foo<bits<8> b> {
+// CHECK: list<int> ListOfInts = [170];
+// CHECK: list<int> AnotherList = [170, 7];
+  list<int> ListOfInts = [!cast<int>(b)];
+  list<int> AnotherList = [!cast<int>(b), !cast<int>({1, 1, 1})];
+}
+
+def : Foo<{1, 0, 1, 0, 1, 0, 1, 0}>;