Silence a compiler warning (#913)

GCC 11 complains about allocsize_0 being uninitialized. I don't see
any place where the uninitialized value is actually used, but let's
silence the compiler warning anyway.

Author: Keno

ir/attrs.h

@@ -136,8 +136,8 @@ class FnAttrs final {
   uint64_t derefOrNullBytes = 0; // DereferenceableOrNull
   uint64_t align = 0;
 
-  unsigned allocsize_0;
-  unsigned allocsize_1 = -1u;
+  unsigned allocsize_0 = 0;      // AllocSize
+  unsigned allocsize_1 = -1u;    // AllocSize
 
   MemoryAccess mem;