Merging r328755:

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r328755 | gbiv | 2018-03-28 20:12:03 -0700 (Wed, 28 Mar 2018) | 10 lines

[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)`).

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llvm-svn: 329670