[TableGen] Fix spurious type error in bit assignment.

Summary:
The following example gives the error message "expected value of type
'bits<32>', got 'bit'" on the assignment.

    class Instruction { bits<32> encoding; }
    def foo: Instruction { let encoding{10} = !eq(0, 1); }

But there's nothing wrong with this code: 'bit' is a perfectly good
type for the RHS of an assignment to a //single bit// of an
instruction encoding.

The problem is that `ParseBodyItem` is accidentally type-checking the
RHS against the full type of the `encoding` field, without adjusting
it in the case where we're only assigning to a subset of the bits. The
fix is trivial.

Reviewers: nhaehnle, hfinkel

Reviewed By: hfinkel

Subscribers: hiraditya, llvm-commits

Tags: #llvm

Differential Revision: https://reviews.llvm.org/D74220

lib/TableGen/TGParser.cpp

@@ -2640,6 +2640,11 @@ bool TGParser::ParseBodyItem(Record *CurRec) {
     return TokError("Value '" + FieldName->getValue() + "' unknown!");
 
   RecTy *Type = Field->getType();
+  if (!BitList.empty() && isa<BitsRecTy>(Type)) {
+    // When assigning to a subset of a 'bits' object, expect the RHS to have
+    // the type of that subset instead of the type of the whole object.
+    Type = BitsRecTy::get(BitList.size());
+  }
 
   Init *Val = ParseValue(CurRec, Type);
   if (!Val) return true;

test/TableGen/BitsInit.td

@@ -56,6 +56,10 @@ def {
   // Make sure we can initialise ints with bits<> values.
   int J = H;
   int K = { 0, 1 };
+
+  bits<2> L;
+  let L{0} = 1;
+  let L{1} = !eq(L{0}, 0);
 }
 
 // CHECK: def {{.*}} {
@@ -82,4 +86,5 @@ def {
 // CHECK: bits<16> I = { 0, 1, 1, 0, 0, 1, 0, 0, 0, 1, 1, 0, 0, 1, 0, 0 };
 // CHECK: int J = 52275;
 // CHECK: int K = 1;
+// CHECK: bits<2> L = { 0, 1 };
 // CHECK: }