when: emit switch for String if possible

Effectively, the following when structure:

  when (s) {
    s1, s2 -> e1,
    s3 -> e2,
    s4 -> e3,
    ...
    else -> e
  }

is implemented as:

  when (s.hashCode()) {
    h1 -> {
      if (s == s1)
        e1
      else if (s == s2)
        e1
      else if (s == s3)
        e2
      else
        e
    }
    h2 -> if (s == s3) e2 else e,
    ...
    else -> e
  }

where s1.hashCode() == s2.hashCode() == s3.hashCode() == h1,
      s4.hashCode() == h2.

A tableswitch or lookupswitch is used for the hash code lookup.

Change-Id: I087bf623dbb4a41d3cc64399a1b42342a50757a6

compiler/testData/codegen/bytecodeText/whenStringOptimization/sameHashCode.kt

@@ -1,4 +1,3 @@
-// IGNORE_BACKEND: JVM_IR
 fun foo(x : String) : String {
     assert("abz]".hashCode() == "aby|".hashCode())