[InstCombine] Shift amount reassociation: fixup constantexpr handling (PR42484)

I was actually wondering if there was some nicer way than m_Value()+cast,
but apparently what i was really "subconsciously" thinking about
was correctness issue.

hasNoUnsignedWrap()/hasNoUnsignedWrap() exist for Instruction,
not for BinaryOperator, so let's just use m_Instruction(),
thus both avoiding a cast, and a crash.

Fixes https://bugs.llvm.org/show_bug.cgi?id=42484,
      https://bugs.chromium.org/p/oss-fuzz/issues/detail?id=15587

llvm-svn: 364915

llvm/lib/Transforms/InstCombine/InstCombineShifts.cpp

@@ -29,12 +29,12 @@ static Instruction *
 reassociateShiftAmtsOfTwoSameDirectionShifts(BinaryOperator *Sh0,
                                              const SimplifyQuery &SQ) {
   // Look for:  (x shiftopcode ShAmt0) shiftopcode ShAmt1
-  Value *X, *ShAmt1, *Sh1Value, *ShAmt0;
+  Value *X, *ShAmt1, *ShAmt0;
+  Instruction *Sh1;
   if (!match(Sh0, m_Shift(m_CombineAnd(m_Shift(m_Value(X), m_Value(ShAmt1)),
-                                       m_Value(Sh1Value)),
+                                       m_Instruction(Sh1)),
                           m_Value(ShAmt0))))
     return nullptr;
-  auto *Sh1 = cast<BinaryOperator>(Sh1Value);
 
   // The shift opcodes must be identical.
   Instruction::BinaryOps ShiftOpcode = Sh0->getOpcode();

llvm/test/Transforms/InstCombine/shift-amount-reassociation.ll

@@ -154,6 +154,21 @@ define i32 @t11_shl_nsw_flag_preservation(i32 %x, i32 %y) {
   ret i32 %t3
 }
 
+; Reduced from https://bugs.chromium.org/p/oss-fuzz/issues/detail?id=15587
+@X = external global i32
+define i64 @constantexpr() {
+; CHECK-LABEL: @constantexpr(
+; CHECK-NEXT:    ret i64 0
+;
+  %A = alloca i64
+  %L = load i64, i64* %A
+  %V = add i64 ptrtoint (i32* @X to i64), 0
+  %B2 = shl i64 %V, 0
+  %B4 = ashr i64 %B2, %L
+  %B = and i64 undef, %B4
+  ret i64 %B
+}
+
 ; No one-use tests since we will only produce a single instruction here.
 
 ; Negative tests