[SR-990] Unable to eliminate force cast with known type

[ddunbar]

Previous ID	SR-990
Radar	None
Original Reporter	@ddunbar
Type	Bug

Additional Detail from JIRA

Votes	0
Component/s	Compiler
Labels	Bug, Performance
Assignee	None
Priority	Medium

md5: d4c71e79aeff2734b27fd87267021530

Issue Description:

Swift 3 is unable to optimize f1() down to the trivial function here, even though it should be statically inferable that the cast always succeeds:

private protocol P {
    static func foo() -> P
}
private extension P {
    static func foo2<T: P>() -> T {
        return foo() as! T
    }
}

private class A: P {
    func hi() {}
    
    static func foo() -> P {
        return A()
    }
}
private class B: A {
}

private func f0<T: P>(type: T.Type) -> T {
    return T.self.foo2()
}

func f1() {
    f0(A.self).hi()
}

I only tested with an older Swift 3:
Apple Swift version 3.0-dev (LLVM a7663bb722, Clang 4ca3c7fa28, Swift 1c2f40e)