Fixes type assertion argument in type predicate functions bug.

[tinganho]

Ref: #3812.

[JsonFreeman]

This is the same check as above, it's just doing the check on a different expression. I would instead structure it like this:

let arg = expr.arguments[signature.typePredicate.parameterIndex];
if (arg.kind === SyntaxKind.AsExpression || arg.kind === SyntaxKind.TypeAssertionExpression) {
    arg = (<AssertionExpression>arg).expression;
}
if (arg.kind === SyntaxKind.Identifier && getSymbolAtLocation(arg) === symbol) {
    if (!assumeTrue) ...
}

That way you do not need the narrowType inner function

[JsonFreeman]

Something I don't get here. Why would you call a type predicate function and cast its input? Presumably if you are casting, you know what type it is already.

[tinganho]

I was probably to fast on making a fix without even thinking about it. It doesn't make any sense at all.