base: Introduce a version of reverseBits for 8 bit types.

These types shouldn't be shifted by 8, since shifting a type by its
width is technically undefined behavior. We never actually use the
result from this shift, but it still upsets certain versions of clang.

Change-Id: I425431473fa44a6e0de2edf780c265ff4e3f440e
Reviewed-on: https://gem5-review.googlesource.com/c/public/gem5/+/41593
Reviewed-by: Gabe Black <gabe.black@gmail.com>
Reviewed-by: Jason Lowe-Power <power.jg@gmail.com>
Maintainer: Gabe Black <gabe.black@gmail.com>
Tested-by: kokoro <noreply+kokoro@google.com>

src/base/bitfield.hh

@@ -209,7 +209,7 @@ replaceBits(T& val, unsigned bit, B bit_val)
  * @ingroup api_bitfield
  */
 template <class T>
-std::enable_if_t<std::is_integral<T>::value, T>
+std::enable_if_t<std::is_integral<T>::value && sizeof(T) != 1, T>
 reverseBits(T val, size_t size=sizeof(T))
 {
     assert(size <= sizeof(T));
@@ -223,6 +223,14 @@ reverseBits(T val, size_t size=sizeof(T))
     return output;
 }
 
+template <class T>
+std::enable_if_t<std::is_integral<T>::value && sizeof(T) == 1, T>
+reverseBits(T val, size_t size=sizeof(T))
+{
+    assert(size == 1);
+    return reverseBitsLookUpTable[val];
+}
+
 /**
  * Returns the bit position of the MSB that is set in the input
  *