[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/Writer/BitcodeWriter.cpp

@@ -667,6 +667,8 @@ static uint64_t getAttrKindEncoding(Attribute::AttrKind Kind) {
     return bitc::ATTR_KIND_NOCF_CHECK;
   case Attribute::NoUnwind:
     return bitc::ATTR_KIND_NO_UNWIND;
+  case Attribute::NullPointerIsValid:
+    return bitc::ATTR_KIND_NULL_POINTER_IS_VALID;
   case Attribute::OptForFuzzing:
     return bitc::ATTR_KIND_OPT_FOR_FUZZING;
   case Attribute::OptimizeForSize: