Fix nonnull-compare warnings

The following warnings are emitted when compiling with gcc without -fno-delete-null-pointer-checks:

In file included from .../src/hotspot/share/memory/allocation.hpp:29,
                 from .../src/hotspot/share/classfile/classLoaderData.hpp:28,
                 from .../src/hotspot/share/precompiled/precompiled.hpp:34:
ad_x86_pipeline.cpp: In member function 'uint Pipeline::operand_latency(uint, const Pipeline*) const':
.../src/hotspot/share/utilities/debug.hpp:152:7: warning: nonnull argument 'this' compared to NULL [-Wnonnull-compare]
  152 | if (! VMASSERT_CHECK_PASSED(p)) { \
      | ^
.../src/hotspot/share/utilities/debug.hpp:160:24: note: in expansion of macro 'vmassert'
  160 | #define assert(p, ...) vmassert(p, __VA_ARGS__)
      | ^~~~~~~~
ad_x86_pipeline.cpp:142:3: note: in expansion of macro 'assert'

.../src/hotspot/share/asm/codeBuffer.cpp: In member function 'void CodeBuffer::print()':
.../src/hotspot/share/asm/codeBuffer.cpp:1064:3: warning: nonnull argument 'this' compared to NULL [-Wnonnull-compare]
 1064 | if (this == nullptr) {
      | ^~