[InstCombine] Prevent infinite loop with two shifts

The following pattern: `(C2 << X) << C1` will usually be
transformed into `(C2 << C1) << X`, essentially swapping `X` and `C1`.

However, this should not only done when `C1` is an immediate constant, otherwise this
can lead to both constants being swapped forever

This fixes llvm#118798

llvm/lib/Transforms/InstCombine/InstCombineShifts.cpp

@@ -427,7 +427,8 @@ Instruction *InstCombinerImpl::commonShiftTransforms(BinaryOperator &I) {
       if (Instruction *R = FoldOpIntoSelect(I, SI))
         return R;
 
-  if (Constant *CUI = dyn_cast<Constant>(Op1))
+  Constant *CUI;
+  if (match(Op1, m_ImmConstant(CUI)))
     if (Instruction *Res = FoldShiftByConstant(Op0, CUI, I))
       return Res;
 

llvm/test/Transforms/InstCombine/shl-twice-constant.ll

@@ -0,0 +1,16 @@
+; NOTE: Assertions have been autogenerated by utils/update_test_checks.py
+; RUN: opt < %s -passes=instcombine -S | FileCheck %s
+
+@c = external constant i8
+@c2 = external constant i8
+
+define i64 @testfunc() {
+; CHECK-LABEL: @testfunc(
+; CHECK-NEXT:    [[SHL1:%.*]] = shl nuw i64 1, ptrtoint (ptr @c2 to i64)
+; CHECK-NEXT:    [[SHL2:%.*]] = shl i64 [[SHL1]], ptrtoint (ptr @c to i64)
+; CHECK-NEXT:    ret i64 [[SHL2]]
+;
+  %shl1 = shl i64 1, ptrtoint (ptr @c2 to i64)
+  %shl2 = shl i64 %shl1, ptrtoint (ptr @c to i64)
+  ret i64 %shl2
+}