[FIR2IR] Set correct endOffset for the IrBranch

We want to use end offset of branch's result as the end for the whole
branch, not just the end of its condition.

#KT-63779 Fixed

compiler/testData/ir/sourceRanges/kt63779_2.kt

@@ -0,0 +1,5 @@
+// In this test K2 behaviour is preferable.
+fun topLevelMethod() {
+    var z = 3
+    if(true) { z = 4 } else { z = 5 }
+}