c++: Readd [LR]ROTATE_EXPR support to constexpr.c [PR94067]

Since r10-6527-gaaa26bf4 fold_for_warn
will perform maybe_constant_value even on some cp_fold produced trees and
so can include rotate exprs which were removed last fall from constexpr.c

2020-03-09  Jakub Jelinek  <jakub@redhat.com>

	PR c++/94067
	Revert
	2019-10-11  Paolo Carlini  <paolo.carlini@oracle.com>

	* constexpr.c (cxx_eval_constant_expression): Do not handle
	RROTATE_EXPR and LROTATE_EXPR.

	* g++.dg/warn/Wconversion-pr94067.C: New test.

gcc/cp/ChangeLog

+2020-03-09  Jakub Jelinek  <jakub@redhat.com>
+
+	PR c++/94067
+	Revert
+	2019-10-11  Paolo Carlini  <paolo.carlini@oracle.com>
+
+	* constexpr.c (cxx_eval_constant_expression): Do not handle
+	RROTATE_EXPR and LROTATE_EXPR.
+
 2020-03-09  Marek Polacek  <polacek@redhat.com>
 
 	PR c++/94050 - ABI issue with alignas on armv7hl.

gcc/cp/constexpr.c

@@ -5730,6 +5730,8 @@ cxx_eval_constant_expression (const constexpr_ctx *ctx, tree t,
     case MAX_EXPR:
     case LSHIFT_EXPR:
     case RSHIFT_EXPR:
+    case LROTATE_EXPR:
+    case RROTATE_EXPR:
     case BIT_IOR_EXPR:
     case BIT_XOR_EXPR:
     case BIT_AND_EXPR:
@@ -7853,6 +7855,8 @@ potential_constant_expression_1 (tree t, bool want_rval, bool strict, bool now,
     case MAX_EXPR:
     case LSHIFT_EXPR:
     case RSHIFT_EXPR:
+    case LROTATE_EXPR:
+    case RROTATE_EXPR:
     case BIT_IOR_EXPR:
     case BIT_XOR_EXPR:
     case BIT_AND_EXPR:

gcc/testsuite/ChangeLog

+2020-03-09  Jakub Jelinek  <jakub@redhat.com>
+
+	PR c++/94067
+	* g++.dg/warn/Wconversion-pr94067.C: New test.
+
 2020-03-09  Marek Polacek  <polacek@redhat.com>
 
 	PR c++/94050 - ABI issue with alignas on armv7hl.

gcc/testsuite/g++.dg/warn/Wconversion-pr94067.C

+// PR c++/94067
+// { dg-do compile }
+// { dg-options "-Wconversion" }
+
+static inline unsigned short
+swap (unsigned short x)
+{
+  return (x >> 8) | static_cast<unsigned short>(x << 8);
+}