[IR] Convert null-pointer-is-valid into an enum attribute

The "null-pointer-is-valid" attribute needs to be checked by many
pointer-related combines. To make the check more efficient, convert
it from a string into an enum attribute.

In the future, this attribute may be replaced with data layout
properties.

Differential Revision: https://reviews.llvm.org/D78862

llvm/lib/Bitcode/Reader/BitcodeReader.cpp

@@ -1469,6 +1469,8 @@ static Attribute::AttrKind getAttrFromCode(uint64_t Code) {
     return Attribute::NoCfCheck;
   case bitc::ATTR_KIND_NO_UNWIND:
     return Attribute::NoUnwind;
+  case bitc::ATTR_KIND_NULL_POINTER_IS_VALID:
+    return Attribute::NullPointerIsValid;
   case bitc::ATTR_KIND_OPT_FOR_FUZZING:
     return Attribute::OptForFuzzing;
   case bitc::ATTR_KIND_OPTIMIZE_FOR_SIZE:
@@ -1654,7 +1656,7 @@ Error BitcodeReader::parseAttributeGroupBlock() {
         }
       }
 
-      UpgradeFramePointerAttributes(B);
+      UpgradeAttributes(B);
       MAttributeGroups[GrpID] = AttributeList::get(Context, Idx, B);
       break;
     }