[InstCombine] Add test for iterator invalidation bug; NFC

Author: nikic

llvm/test/Transforms/InstCombine/bitcast-phi-uselistorder.ll

@@ -0,0 +1,36 @@
+; NOTE: Assertions have been autogenerated by utils/update_test_checks.py
+; RUN: opt -S -instcombine < %s | FileCheck %s
+
+@Q = internal unnamed_addr global double 1.000000e+00, align 8
+
+define double @test(i1 %c, i64* %p) {
+; CHECK-LABEL: @test(
+; CHECK-NEXT:  entry:
+; CHECK-NEXT:    br i1 [[C:%.*]], label [[IF:%.*]], label [[END:%.*]]
+; CHECK:       if:
+; CHECK-NEXT:    [[LOAD:%.*]] = load i64, i64* bitcast (double* @Q to i64*), align 8
+; CHECK-NEXT:    [[TMP0:%.*]] = bitcast i64 [[LOAD]] to double
+; CHECK-NEXT:    [[PHITMP:%.*]] = bitcast i64 [[LOAD]] to double
+; CHECK-NEXT:    br label [[END]]
+; CHECK:       end:
+; CHECK-NEXT:    [[TMP1:%.*]] = phi double [ 0.000000e+00, [[ENTRY:%.*]] ], [ [[TMP0]], [[IF]] ]
+; CHECK-NEXT:    [[PHI:%.*]] = phi double [ 0.000000e+00, [[ENTRY]] ], [ [[PHITMP]], [[IF]] ]
+; CHECK-NEXT:    [[TMP2:%.*]] = bitcast i64* [[P:%.*]] to double*
+; CHECK-NEXT:    store double [[TMP1]], double* [[TMP2]], align 8
+; CHECK-NEXT:    ret double [[PHI]]
+;
+entry:
+  br i1 %c, label %if, label %end
+
+if:
+  %load = load i64, i64* bitcast (double* @Q to i64*), align 8
+  br label %end
+
+end:
+  %phi = phi i64 [ 0, %entry ], [ %load, %if ]
+  store i64 %phi, i64* %p, align 8
+  %cast = bitcast i64 %phi to double
+  ret double %cast
+
+  uselistorder i64 %phi, { 1, 0 }
+}