[InstCombine] Require ImmConstant in shift of shift fold

This fixes an infinite loop reported at:
82f68a9#commitcomment-132406739

Author: nikic

llvm/lib/Transforms/InstCombine/InstCombineShifts.cpp

@@ -760,7 +760,7 @@ Instruction *InstCombinerImpl::FoldShiftByConstant(Value *Op0, Constant *C1,
   // (C2 >> X) >> C1 --> (C2 >> C1) >> X
   Constant *C2;
   Value *X;
-  if (match(Op0, m_BinOp(I.getOpcode(), m_Constant(C2), m_Value(X))))
+  if (match(Op0, m_BinOp(I.getOpcode(), m_ImmConstant(C2), m_Value(X))))
     return BinaryOperator::Create(
         I.getOpcode(), Builder.CreateBinOp(I.getOpcode(), C2, C1), X);
 

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

@@ -732,3 +732,15 @@ define <2 x i8> @lshr_shl_demand5_nonuniform_vec_both(<2 x i8> %x) {
   %r = and <2 x i8> %shl, <i8 -4, i8 -16>
   ret <2 x i8> %r
 }
+
+@g = external global i8, align 8
+
+define i64 @ashr_ashr_constexpr() {
+; CHECK-LABEL: @ashr_ashr_constexpr(
+; CHECK-NEXT:    [[SHR2:%.*]] = ashr exact i64 ptrtoint (ptr @g to i64), 3
+; CHECK-NEXT:    ret i64 [[SHR2]]
+;
+  %shr = ashr i64 ptrtoint (ptr @g to i64), 1
+  %shr2 = ashr i64 %shr, 2
+  ret i64 %shr2
+}