[FIR] Don't capture flexible types that only have projections in one bound

This fixes a false positive NEW_INFERENCE_ERROR.
See the code comment for the rationale.

#KT-63982 Fixed

compiler/testData/diagnostics/tests/inference/kt63982.kt

@@ -0,0 +1,19 @@
+// ISSUE: KT-63982
+// FIR_IDENTICAL
+// DIAGNOSTICS: -DEBUG_INFO_SMARTCAST
+
+// FILE: StubBasedPsiElement.java
+public interface StubBasedPsiElement<Stub extends StubElement> extends PsiElement {
+    Stub getStub();
+}
+
+// FILE: test.kt
+interface PsiElement
+interface JSCallExpression: PsiElement
+interface StubElement<T : PsiElement>
+
+fun test(child: PsiElement) {
+    if (child is JSCallExpression) {
+        val callStub = if (child is StubBasedPsiElement<*>) child.stub else null
+    }
+}