Fix PostDec operator (#361)

* Fix PostDec operator

* Add test: decrement operator

tools/cgeist/Lib/clang-mlir.cc

@@ -2335,7 +2335,7 @@ ValueCategory MLIRScanner::VisitUnaryOperator(clang::UnaryOperator *U) {
     }
     sub.store(loc, builder, next);
     return ValueCategory(
-        (U->getOpcode() == clang::UnaryOperator::Opcode::UO_PostInc) ? prev
+        (U->getOpcode() == clang::UnaryOperator::Opcode::UO_PostDec) ? prev
                                                                      : next,
         /*isReference*/ false);
   }

tools/cgeist/Test/Verification/decrement.c

@@ -0,0 +1,21 @@
+// RUN: cgeist %s --function=* -S | FileCheck %s
+
+int prefix_decrement(int x)
+{
+    return --x;
+}
+
+int postfix_decrement(int x)
+{
+    return x--;
+}
+
+// CHECK: func.func @prefix_decrement(%[[arg0:.+]]: i32) -> i32 attributes {llvm.linkage = #llvm.linkage<external>} {
+// CHECK-NEXT:   %[[c1_i32:.+]] = arith.constant -1 : i32
+// CHECK-NEXT:   %[[V0:.+]] = arith.addi %[[arg0]], %[[c1_i32]] : i32
+// CHECK-NEXT:   return %[[V0]] : i32
+// CHECK-NEXT: }
+
+// CHECK: func.func @postfix_decrement(%[[arg0:.+]]: i32) -> i32 attributes {llvm.linkage = #llvm.linkage<external>} {
+// CHECK-NEXT:   return %[[arg0]] : i32
+// CHECK-NEXT: }