[c++] suppress redundant null-addr warn in pfn from pmfn

[c++] suppress redundant null-addr warn in pfn from pmfn

When TARGET_PTRMEMFUNC_VBIT_LOCATION == ptrmemfunc_vbit_in_delta, when
we warn about comparing a pointer-to-member-function with NULL, we
also warn about comparing the pointer-to-function extracted from it
with NULL, which is redundant.  Suppress the redundant warning.

for  gcc/cp/ChangeLog

	* typeck.cc (cp_build_binary_op): Suppress redundant warning
	for pfn null test in pmfn test with vbit-in-delta.