indvars: insert truncate at loop boundary to avoid redundant IVs.

When widening an IV to remove s/zext, we generally try to eliminate
the original narrow IV. However, LCSSA phi nodes outside the loop were
still using the original IV. Clean this up more aggressively to avoid
redundancy in generated code.

git-svn-id: https://llvm.org/svn/llvm-project/llvm/trunk@198338 91177308-0d34-0410-b5e6-96231b3b80d8

test/Transforms/IndVarSimplify/iv-widen.ll

@@ -0,0 +1,39 @@
+; RUN: opt < %s -indvars -S | FileCheck %s
+
+target triple = "x86_64-apple-darwin"
+
+; CHECK-LABEL: @sloop
+; CHECK-LABEL: B18:
+; Only one phi now.
+; CHECK: phi
+; CHECK-NOT: phi
+; We now get 2 trunc, one for the gep and one for the lcssa phi.
+; CHECK: trunc i64 %indvars.iv to i32
+; CHECK: trunc i64 %indvars.iv to i32
+; CHECK-LABEL: B24:
+define void @sloop(i32* %a) {
+Prologue:
+  br i1 undef, label %B18, label %B6
+
+B18:                                        ; preds = %B24, %Prologue
+  %.02 = phi i32 [ 0, %Prologue ], [ %tmp33, %B24 ]
+  %tmp23 = zext i32 %.02 to i64
+  %tmp33 = add i32 %.02, 1
+  %o = getelementptr i32* %a, i32 %.02
+  %v = load i32* %o
+  %t = icmp eq i32 %v, 0
+  br i1 %t, label %exit24, label %B24
+
+B24:                                        ; preds = %B18
+  %t2 = icmp eq i32 %tmp33, 20
+  br i1 %t2, label %B6, label %B18
+
+B6:                                       ; preds = %Prologue
+  ret void
+
+exit24:                      ; preds = %B18
+  call void @dummy(i32 %.02)
+  unreachable
+}
+
+declare void @dummy(i32)