[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/safeCallWithIncrementDecrement.fir.kt.txt

@@ -1,66 +0,0 @@
-package test
-
-class C {
-  constructor() /* primary */ {
-    super/*Any*/()
-    /* <init>() */
-
-  }
-
-}
-
-var C?.p: Int
-  get(): Int {
-    return 42
-  }
-  set(value: Int) {
-  }
-
-operator fun Int?.inc(): Int? {
-  return { // BLOCK
-    val tmp0_safe_receiver: Int? = <this>
-    when {
-      EQEQ(arg0 = tmp0_safe_receiver, arg1 = null) -> null
-      else -> tmp0_safe_receiver.inc()
-    }
-  }
-}
-
-operator fun Int?.get(index: Int): Int {
-  return 42
-}
-
-operator fun Int?.set(index: Int, value: Int) {
-}
-
-fun testProperty(nc: C?) {
-  val <unary>: Int? = { // BLOCK
-    val tmp1_safe_receiver: C? = nc
-    when {
-      EQEQ(arg0 = tmp1_safe_receiver, arg1 = null) -> null
-      else -> tmp1_safe_receiver.<get-p>()
-    }
-  }
-  { // BLOCK
-    val tmp2_safe_receiver: C? = nc
-    when {
-      EQEQ(arg0 = tmp2_safe_receiver, arg1 = null) -> null
-      else -> tmp2_safe_receiver.<set-p>(value = <unary>.inc())
-    }
-  } /*~> Unit */
-  <unary> /*~> Unit */
-}
-
-fun testArrayAccess(nc: C?) {
-  val <array>: Int? = { // BLOCK
-    val tmp3_safe_receiver: C? = nc
-    when {
-      EQEQ(arg0 = tmp3_safe_receiver, arg1 = null) -> null
-      else -> tmp3_safe_receiver.<get-p>()
-    }
-  }
-  val <index0>: Int = 0
-  val <unary>: Int = <array>.get(index = <index0>)
-  <array>.set(index = <index0>, value = <unary>.inc())
-  <unary> /*~> Unit */
-}