Fix SILCombine to devirtualize existentials wrapped in enum.
This allows Swift code to implement a fast path via a protocol type
check as follows:
if let existentialVal = genericVal as? SomeProtocol {
// do something fast.
}
Fixes <rdar://problem/46322928> Failure to devirtualize a protocol
method applied to an opened existential blocks implemention of
DataProtocol.
Note: the approach of devirtualization via backward pattern matching
is fundamentally wrong and will never be fully general. It should be a
forward type propagation.
