[InstCombine] dropRedundantMaskingOfLeftShiftInput(): check that adding shift amounts doesn't overflow (PR49778)

This is identical to 781d077,
but for the other function.

For certain shift amount bit widths, we must first ensure that adding
shift amounts is safe, that the sum won't have an unsigned overflow.

Fixes https://bugs.llvm.org/show_bug.cgi?id=49778

(cherry picked from commit 2760a80)

Author: LebedevRI

llvm/lib/Transforms/InstCombine/InstCombineShifts.cpp

@@ -226,9 +226,9 @@ dropRedundantMaskingOfLeftShiftInput(BinaryOperator *OuterShift,
     // Peek through an optional zext of the shift amount.
     match(MaskShAmt, m_ZExtOrSelf(m_Value(MaskShAmt)));
 
-    // We have two shift amounts from two different shifts. The types of those
-    // shift amounts may not match. If that's the case let's bailout now.
-    if (MaskShAmt->getType() != ShiftShAmt->getType())
+    // Verify that it would be safe to try to add those two shift amounts.
+    if (!canTryToConstantAddTwoShiftAmounts(OuterShift, ShiftShAmt, Masked,
+                                            MaskShAmt))
       return nullptr;
 
     // Can we simplify (MaskShAmt+ShiftShAmt) ?
@@ -258,9 +258,9 @@ dropRedundantMaskingOfLeftShiftInput(BinaryOperator *OuterShift,
     // Peek through an optional zext of the shift amount.
     match(MaskShAmt, m_ZExtOrSelf(m_Value(MaskShAmt)));
 
-    // We have two shift amounts from two different shifts. The types of those
-    // shift amounts may not match. If that's the case let's bailout now.
-    if (MaskShAmt->getType() != ShiftShAmt->getType())
+    // Verify that it would be safe to try to add those two shift amounts.
+    if (!canTryToConstantAddTwoShiftAmounts(OuterShift, ShiftShAmt, Masked,
+                                            MaskShAmt))
       return nullptr;
 
     // Can we simplify (ShiftShAmt-MaskShAmt) ?

llvm/test/Transforms/InstCombine/redundant-left-shift-input-masking-pr49778.ll

@@ -4,7 +4,12 @@
 ; PR49778: this should not be folded to 0.
 define i32 @src(i1 %x2) {
 ; CHECK-LABEL: @src(
-; CHECK-NEXT:    ret i32 0
+; CHECK-NEXT:    [[X13:%.*]] = zext i1 [[X2:%.*]] to i32
+; CHECK-NEXT:    [[_7:%.*]] = shl i32 -1, [[X13]]
+; CHECK-NEXT:    [[MASK:%.*]] = xor i32 [[_7]], -1
+; CHECK-NEXT:    [[_8:%.*]] = and i32 [[MASK]], [[X13]]
+; CHECK-NEXT:    [[_9:%.*]] = shl i32 [[_8]], [[X13]]
+; CHECK-NEXT:    ret i32 [[_9]]
 ;
   %x13 = zext i1 %x2 to i32
   %_7 = shl i32 4294967295, %x13