[FIR] Save IR dumps to .ir.txt files instead of .txt in tests

This is needed to avoid clashes between different dumps from different
  handlers

compiler/testData/ir/irText/expressions/primitiveComparisons.kt.txt

@@ -93,4 +93,3 @@ fun dtest3(a: Double, b: Double): Boolean {
 fun dtest4(a: Double, b: Double): Boolean {
   return lessOrEqual(arg0 = a, arg1 = b)
 }
-