Switch to select optimization for two-case switches

This is the same optimization of r219233 with modifications to support PHIs with multiple incoming edges from the same block
and a test to check that this condition is handled.

git-svn-id: https://llvm.org/svn/llvm-project/llvm/trunk@219656 91177308-0d34-0410-b5e6-96231b3b80d8

test/Transforms/SimplifyCFG/switch-to-select-multiple-edge-per-block-phi.ll

@@ -0,0 +1,40 @@
+; RUN: opt < %s -simplifycfg -S | FileCheck %s
+
+; a, b;
+; fn1() {
+;   if (b)
+;     if (a == 0 || a == 5)
+;       return a;
+;   return 0;
+; }
+
+; Checking that we handle correctly the case when we have a switch
+; branching multiple times to the same block
+
+@b = common global i32 0, align 4
+@a = common global i32 0, align 4
+
+; Function Attrs: nounwind
+define i32 @fn1() {
+; CHECK-LABEL: @fn1
+; CHECK: %switch.selectcmp1 = icmp eq i32 %1, 5
+; CHECK: %switch.select2 = select i1 %switch.selectcmp1, i32 5, i32 %switch.select
+entry:
+  %0 = load i32* @b, align 4
+  %tobool = icmp eq i32 %0, 0
+  br i1 %tobool, label %if.end3, label %if.then
+
+if.then:
+  %1 = load i32* @a, align 4
+  switch i32 %1, label %if.end3 [
+    i32 5, label %return
+    i32 0, label %return
+  ]
+
+if.end3:
+  br label %return
+
+return:
+  %retval.0 = phi i32 [ 0, %if.end3 ], [ %1, %if.then ], [ %1, %if.then ]
+  ret i32 %retval.0
+}