[IR Parser] Fix a crash handling zero width integer attributes.

[lattner]

Summary

llvm::APInt cannot hold zero bit values, therefore we shouldn't try
to form them.

[lattner]

Thx for the quick review!

mlir/lib/Parser/AttributeParser.cpp

static Optional<APInt> buildAttributeAPInt(Type type, bool isNegative,
   APInt result;
   bool isHex = spelling.size() > 1 && spelling[1] == 'x';
   if (spelling.getAsInteger(isHex ? 0 : 10, result))
     return llvm::None;
 
   // Extend or truncate the bitwidth to the right size.
   unsigned width = type.isIndex() ? IndexType::kInternalStorageBitWidth
                                   : type.getIntOrFloatBitWidth();
+
+  // APInt cannot hold a zero bit value.
+  if (width == 0)
+    return llvm::None;
+
   if (width > result.getBitWidth()) {
     result = result.zext(width);
   } else if (width < result.getBitWidth()) {
     // The parser can return an unnecessarily wide result with leading zeros.
     // This isn't a problem, but truncating off bits is bad.
     if (result.countLeadingZeros() < result.getBitWidth() - width)
       return llvm::None;
 

mlir/test/IR/invalid-ops.mlir

 
 func @subtensor_wrong_static_type(%t: tensor<8x16x4xf32>, %idx : index) {
       // expected-error @+1 {{expected result type to be 'tensor<?x3x?xf32>' or a rank-reduced version. (mismatch of result sizes)}}
   %0 = subtensor %t[0, 0, 0][%idx, 3, %idx][1, 1, 1]
     : tensor<8x16x4xf32> to tensor<4x4x4xf32>
 
   return
 }
+
+// -----
+
+func @no_zero_bit_integer_attrs() {
+  // expected-error @+1 {{integer constant out of range for attribute}}
+  %x = "some.op"(){value = 0 : i0} : () -> f32
+  return
+}
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