optimizing: Fix a potential nullptr dereference

Operator precedence in C++ means the DCHECK was parsed as
DCHECK((allocation != nullptr && allocation->IsNewInstance()) ||
       allocation->IsNewArray())

Caught by the static analyzer:
art/compiler/optimizing/instruction_builder.cc:1004:14: warning: Called
C++ object pointer is null [clang-analyzer-core.CallAndMessage]

Bug: None
Test: Reran the static analyzer. No more nullptr complaints. m builds
the file fine, as well.
Change-Id: Iad94719fc013f1883746128e066452994740f171