Upcast and downcast are switched

[danaugrs]

Hi! Thanks for building AssemblyScript!
I noticed that "upcast" and "downcast" are switched:

assemblyscript/src/compiler.ts

Lines 3472 to 3501 in 53af0e0

	if (fromType.isAssignableTo(toType)) { // downcast or same
	assert(toType.isExternalReference || fromType.kind == toType.kind);
	this.currentType = toType;
	return expr;
	}
	if (explicit && toType.nonNullableType.isAssignableTo(fromType)) { // upcast
	// <Cat | null>(<Animal>maybeCat)
	if (toType.isExternalReference) {
	this.error(
	DiagnosticCode.Not_implemented_0,
	reportNode.range,
	"ref.cast"
	);
	this.currentType = toType;
	return module.unreachable();
	}
	assert(fromType.kind == toType.kind);
	if (!this.options.noAssert) {
	expr = this.makeRuntimeUpcastCheck(expr, fromType, toType, reportNode);
	}
	this.currentType = toType;
	return expr;
	}
	this.error(
	DiagnosticCode.Type_0_is_not_assignable_to_type_1,
	reportNode.range, fromType.toString(), toType.toString()
	);
	this.currentType = toType;
	return module.unreachable();
	}

"Upcast" should be the straightforward, static case whereas "downcast" should be dynamic and more complex.
No logic changes are needed, just the words need to be switched, especially in the error message below:

assemblyscript/src/compiler.ts

Line 10564 in 53af0e0

this.ensureStaticString("unexpected upcast"),

For context, that error message was pointed out to be wrong in LimeChain/demo-subgraph#39.

I created a PR to fix this issue: #2423.

[MaxGraey]

Looks like managed-cast test still fail