JVM don't merge local values in FixStackAnalyzer

We care only about stacks there.
This yields about 10-15% in a pathological case such as KT-41510.

compiler/testData/codegen/box/controlStructures/slowHtmlLikeDsl.kt

@@ -0,0 +1,45 @@
+// TARGET_BACKEND: JVM
+//  ^ this test might be rather slow
+
+class Builder(var content: String)
+
+fun Builder.begin(t: String) {
+    content += "<$t>"
+}
+
+fun Builder.text(t: String) {
+    content += t
+}
+
+fun Builder.end(t: String) {
+    content += "</$t>"
+}
+
+fun err(e: Throwable) {}
+
+inline fun Builder.tag(t: String, body: Builder.() -> Unit) {
+    begin(t)
+    try {
+        body()
+    } catch (e: Throwable) {
+        err(e)
+    } finally {
+        end(t)
+    }
+}
+
+inline fun Builder.t2(body: Builder.() -> Unit) {
+    tag("t", body)
+    tag("t", body)
+}
+
+val expectedLength = 1906
+
+fun box(): String {
+    val b = Builder("")
+    b.t2 { t2 { t2 { t2 { t2 { t2 { t2 { text("1") } } } } } } }
+    if (b.content.length != expectedLength)
+        return "${b.content.length}"
+    else
+        return "OK"
+}