[analyzer] Weaken assert in trackNullOrUndefValue() for property accessors (swift-3.0-branch)
[analyzer] Weaken assert in trackNullOrUndefValue() for property accessors (swift-3.0-branch)

This assert, which was added in r273340, is too strong for swift-3.0-branch

I've weakened it to first strip casts before checking the isa<>().

This is a swift-3.0-branch fixup for rdar://problem/26796666 to make
bots pass when assertions are turned on.

apple/swift-clang,apple/swift-clang,apple/swift-clang,apple/swift-clang,apple/swift-clang,apple/swift-clang,apple/swift-clang,apple/swift-clang,apple/swift-clang,apple/swift-clang