KT-15862 Inline generic functions can unexpectedly box primitives

Previous version of the boxing/unboxing analysis treated merging boxed and non-boxed values as a hazard.
If such merged values are not used (e.g., early return + local variables reused in inlined calls),
corresponding boxing/unboxing operations still can be optimized out.

All information related to boxed value usage by instructions is moved to 'BoxedValueDescriptor'.
Introduce "tainted" (and "clean") boxed values, with the following rules:

  merge(B, B) = B, if unboxed types are compatible,
                T, otherwise

  merge(B, X) = T

  merge(T, X) = T

  where
    X is a non-boxed value,
    B is a "clean" boxed value,
    T is a "tainted" boxed value.

Postpone decision about value merge hazards until a "tainted" value is used.

compiler/testData/codegen/box/boxingOptimization/taintedValues.kt

@@ -0,0 +1,11 @@
+// WITH_RUNTIME
+
+// Just make sure there's no VerifyError
+
+fun getOrElse() =
+        mapOf<String, Int>().getOrElse("foo") { 3 }
+
+fun isNotEmpty(l: ArrayList<Int>) =
+        l.iterator()?.hasNext() ?: false
+
+fun box() = "OK"