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/when/edgeCases.kt

@@ -0,0 +1,10 @@
+fun foo(x: Int): String {
+    return when (x) {
+        2_147_483_647 -> "MAX"
+        -2_147_483_648 -> "MIN"
+        else -> "else"
+    }
+}
+
+// 1 LOOKUPSWITCH
+// 0 TABLESWITCH

compiler/testData/codegen/bytecodeText/when/switchOptimizationDuplicates.kt

@@ -0,0 +1,19 @@
+fun foo(x: Int): Int {
+    return when (x) {
+        1, 1, 2 -> 1001
+        1, 2 -> 1002
+        1 -> 1003
+        2 -> 1004
+        3 -> 1005
+        else -> 1006
+    }
+}
+
+// 1 SIPUSH 1001
+// 0 SIPUSH 1002
+// 0 SIPUSH 1003
+// 0 SIPUSH 1004
+// 1 SIPUSH 1005
+// 1 SIPUSH 1006
+// 1 TABLESWITCH
+// 0 LOOKUPSWITCH