Teach the new SROA a new trick. Now we zap any memcpy or memmoves which

are in fact identity operations. We detect these and kill their
partitions so that even splitting is unaffected by them. This is
particularly important because Clang relies on emitting identity memcpy
operations for struct copies, and these fold away to constants very
often after inlining.

Fixes the last big performance FIXME I have on my plate.

llvm-svn: 165285