[InstCombine] Add test for freeze of PHI with noundef start value (NFC) (#157112)

We should be able to remove this freeze as the incoming values to the
PHI have the same well-defined start value and the GEP can't produce
poison, but this is currently unsupported.

If the freeze is pushed to the incoming values we can remove it:
https://godbolt.org/z/8dE4o1bKf

Author: c-rhodes

llvm/test/Transforms/InstCombine/freeze.ll

@@ -1135,6 +1135,55 @@ exit:
   ret void
 }
 
+; We can remove this freeze as the incoming values to the PHI have the same
+; well-defined start value and the GEP can't produce poison, but this is
+; currently unsupported.
+define void @fold_phi_noundef_start_value(ptr noundef %init, i1 %cond.0, i1 %cond.1, i1 %cond.2) {
+; CHECK-LABEL: define void @fold_phi_noundef_start_value(
+; CHECK-SAME: ptr noundef [[INIT:%.*]], i1 [[COND_0:%.*]], i1 [[COND_1:%.*]], i1 [[COND_2:%.*]]) {
+; CHECK-NEXT:  [[ENTRY:.*]]:
+; CHECK-NEXT:    br label %[[LOOP:.*]]
+; CHECK:       [[LOOP]]:
+; CHECK-NEXT:    [[IV_0:%.*]] = phi ptr [ [[INIT]], %[[ENTRY]] ], [ [[IV_0_NEXT:%.*]], %[[LOOP_LATCH:.*]] ]
+; CHECK-NEXT:    br i1 [[COND_0]], label %[[LOOP_LATCH]], label %[[IF_ELSE:.*]]
+; CHECK:       [[IF_ELSE]]:
+; CHECK-NEXT:    [[IV_1:%.*]] = getelementptr i8, ptr [[IV_0]], i64 -8
+; CHECK-NEXT:    br label %[[LOOP_LATCH]]
+; CHECK:       [[LOOP_LATCH]]:
+; CHECK-NEXT:    [[IV_2:%.*]] = phi ptr [ [[IV_0]], %[[LOOP]] ], [ [[IV_1]], %[[IF_ELSE]] ]
+; CHECK-NEXT:    [[IV_2_FR:%.*]] = freeze ptr [[IV_2]]
+; CHECK-NEXT:    [[IV_2_FR_INT:%.*]] = ptrtoint ptr [[IV_2_FR]] to i64
+; CHECK-NEXT:    [[IV_0_INT:%.*]] = ptrtoint ptr [[IV_0]] to i64
+; CHECK-NEXT:    [[IDX:%.*]] = sub i64 [[IV_0_INT]], [[IV_2_FR_INT]]
+; CHECK-NEXT:    [[IV_0_NEXT]] = getelementptr i8, ptr [[IV_0]], i64 [[IDX]]
+; CHECK-NEXT:    br i1 [[COND_2]], label %[[EXIT:.*]], label %[[LOOP]]
+; CHECK:       [[EXIT]]:
+; CHECK-NEXT:    ret void
+;
+entry:
+  br label %loop
+
+loop:
+  %iv.0 = phi ptr [ %init, %entry ], [ %iv.0.next, %loop.latch ]
+  br i1 %cond.0, label %loop.latch, label %if.else
+
+if.else:
+  %iv.1 = getelementptr i8, ptr %iv.0, i64 -8
+  br label %loop.latch
+
+loop.latch:
+  %iv.2 = phi ptr [ %iv.0, %loop ], [ %iv.1, %if.else ]
+  %iv.2.fr = freeze ptr %iv.2
+  %iv.2.fr.int = ptrtoint ptr %iv.2.fr to i64
+  %iv.0.int = ptrtoint ptr %iv.0 to i64
+  %idx = sub i64 %iv.0.int, %iv.2.fr.int
+  %iv.0.next = getelementptr i8, ptr %iv.0, i64 %idx
+  br i1 %cond.2, label %exit, label %loop
+
+exit:
+  ret void
+}
+
 define void @fold_phi_invoke_start_value(i32 %n) personality ptr undef {
 ; CHECK-LABEL: define void @fold_phi_invoke_start_value(
 ; CHECK-SAME: i32 [[N:%.*]]) personality ptr undef {