[SelectionDAG] Use the correct return type for memcpy, memmove, and memset.

When generating calls to memcpy, memmove, and memset, use void* as the return
type rather than void, to match the standard signatures for these functions.

This has no practical effect for most targets, since the return values of
these calls aren't being used anyway, and most calling conventions tolerate
this kind of mismatch. However, this change will help support future
optimizations to utilize the return value to avoid holding the argument
value live across a call.

llvm-svn: 258691