Add a FIXME in SelectionDAGBuilder before an assert that is valid only on X86.

Add a FIXME in SelectionDAGBuilder before an assert that is valid only on X86.

When lowering memcpy, memset or memmove, this assert checks whether the pointer
operands are in an address space < 256 which means "user defined address space"
on X86.  However, this notion of "user defined address space" does not exist
for other targets.

git-svn-id: https://llvm.org/svn/llvm-project/llvm/trunk@227191 91177308-0d34-0410-b5e6-96231b3b80d8