Fix llvm build after 1b3bef43e3cb, due to API change

After merging llvm commit b9ca73e1a8fd for PR 262608, it would fail to
compile with:

/usr/src/contrib/llvm-project/llvm/lib/IR/Operator.cpp:197:22: error: no member named 'isZero' in 'llvm::APInt'

if (!IndexedSize.isZero()) {
     ~~~~~~~~~~~ ^

Upstream refactored their APInt class, and isZero() was one of the newer
methods which did not yet exist in llvm 13.0.0. Fix this by using the
older but equivalent isNullValue() method instead.

Fixes: 1b3bef43e3cb
MFC after: 3 days

(cherry picked from commit 8e72f458c6d389870730ae0e95d28a0d8609a018)