[MemorySSA] Turn an assert into a condition

Eli pointed out that variadic functions are totally a thing, so this
assert is incorrect.

No test-case is provided, since the only way this assert fires is if a
specific DenseMap falls back to doing `isEqual` checks, and that seems
fairly brittle (and requires a pyramid of growing
`call void (i8, ...) @varargs(i8 0)`).

llvm-svn: 328755