Fix unused variables in IntVect (#4133)

The proposed changes:
- [ ] fix a bug or incorrect behavior in AMReX
- [ ] add new capabilities to AMReX
- [ ] changes answers in the test suite to more than roundoff level
- [ ] are likely to significantly affect the results of downstream AMReX
users
- [ ] include documentation in the code and/or rst files, if appropriate

Src/Base/AMReX_IntVect.H

@@ -55,8 +55,11 @@ public:
             static constexpr unsigned shift1 = sizeof(size_t)>=8 ? 20 : 10;
             static constexpr unsigned shift2 = sizeof(size_t)>=8 ? 40 : 20;
             if constexpr (dim == 1) {
+                amrex::ignore_unused(shift1);
+                amrex::ignore_unused(shift2);
                 return static_cast<std::size_t>(vec[0]);
             } else if constexpr (dim == 2) {
+                amrex::ignore_unused(shift2);
                 return static_cast<std::size_t>(vec[0]) ^
                       (static_cast<std::size_t>(vec[1]) << shift1);
             } else if constexpr (dim == 3) {