[InstCombine] Avoid use of ConstantExpr::getShl()

Use IRBuilder instead. Also use ImmConstant to guarantee that this
will fold.

Author: nikic

llvm/lib/Transforms/InstCombine/InstCombineCasts.cpp

@@ -749,18 +749,18 @@ Instruction *InstCombinerImpl::visitTrunc(TruncInst &Trunc) {
     }
 
     Constant *C;
-    if (match(Src, m_OneUse(m_LShr(m_Value(X), m_Constant(C))))) {
+    if (match(Src, m_OneUse(m_LShr(m_Value(X), m_ImmConstant(C))))) {
       // trunc (lshr X, C) to i1 --> icmp ne (and X, C'), 0
       Constant *One = ConstantInt::get(SrcTy, APInt(SrcWidth, 1));
-      Constant *MaskC = ConstantExpr::getShl(One, C);
+      Value *MaskC = Builder.CreateShl(One, C);
       Value *And = Builder.CreateAnd(X, MaskC);
       return new ICmpInst(ICmpInst::ICMP_NE, And, Zero);
     }
     if (match(Src, m_OneUse(m_c_Or(m_LShr(m_Value(X), m_ImmConstant(C)),
                                    m_Deferred(X))))) {
       // trunc (or (lshr X, C), X) to i1 --> icmp ne (and X, C'), 0
       Constant *One = ConstantInt::get(SrcTy, APInt(SrcWidth, 1));
-      Constant *MaskC = ConstantExpr::getShl(One, C);
+      Value *MaskC = Builder.CreateShl(One, C);
       Value *And = Builder.CreateAnd(X, Builder.CreateOr(MaskC, One));
       return new ICmpInst(ICmpInst::ICMP_NE, And, Zero);
     }