Optimize a ^ ((a ^ b) & mask) to (~mask & a) | (b & mask).

Optimize a ^ ((a ^ b) & mask) to (~mask & a) | (b & mask).

From the perspective of the pipeline, `andn + and + ior` version take
2 cycles(AND and ANDN doesn't have dependence), but xor + and + xor
will take 3 cycles.

-       xorl    %edi, %esi
        andl    %edx, %esi
-       movl    %esi, %eax
-       xorl    %edi, %eax
+       andn    %edi, %edx, %eax
+       orl     %esi, %eax

gcc/ChangeLog:

	PR target/94790
	* config/i386/i386.md (*xor2andn): New define_insn_and_split.

gcc/testsuite/ChangeLog:

	PR target/94790
	* gcc.target/i386/pr94790-1.c: New test.
	* gcc.target/i386/pr94790-2.c: Ditto.