[InstCombine] Combine XOR and AES instructions on ARM/ARM64.

The ARM/ARM64 AESE and AESD instructions have a builtin XOR as the first step in
the instruction. Therefore, if the AES key is zero and the AES data was
previously XORed, it can be combined into a single instruction.

Differential Revision: https://reviews.llvm.org/D47239
Patch by Michael Brase!

llvm-svn: 333193