[base] Simplify static_asserts in span's subview API

This change Simplifies the static_asserts in base::span's fixed subview
API first<Count>(), last<Count>() and subspan<Count, Offset>(). These
simplifications rely on the fact that dynamic_extent is equal to
std::numeric_limits<size_t>::max(), and thus will never be smaller than
any other possible value.

References:
- https://cplusplus.github.io/LWG/issue3103
- https://github.com/cplusplus/draft/commit/6a0aa7

Bug: 828324
Change-Id: I626b7e65b1ae8010485c4387a936704810e43fe5
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/2089843
Auto-Submit: Jan Wilken Dörrie <jdoerrie@chromium.org>
Reviewed-by: Daniel Cheng <dcheng@chromium.org>
Commit-Queue: Daniel Cheng <dcheng@chromium.org>
Cr-Commit-Position: refs/heads/master@{#747494}

base/containers/span.h

@@ -309,16 +309,14 @@ class span : public internal::ExtentStorage<Extent> {
   // [span.sub], span subviews
   template <size_t Count>
   constexpr span<T, Count> first() const noexcept {
-    static_assert(Extent == dynamic_extent || Count <= Extent,
-                  "Count must not exceed Extent");
+    static_assert(Count <= Extent, "Count must not exceed Extent");
     CHECK(Extent != dynamic_extent || Count <= size());
     return {data(), Count};
   }
 
   template <size_t Count>
   constexpr span<T, Count> last() const noexcept {
-    static_assert(Extent == dynamic_extent || Count <= Extent,
-                  "Count must not exceed Extent");
+    static_assert(Count <= Extent, "Count must not exceed Extent");
     CHECK(Extent != dynamic_extent || Count <= size());
     return {data() + (size() - Count), Count};
   }
@@ -330,10 +328,8 @@ class span : public internal::ExtentStorage<Extent> {
                       : (Extent != dynamic_extent ? Extent - Offset
                                                   : dynamic_extent))>
   subspan() const noexcept {
-    static_assert(Extent == dynamic_extent || Offset <= Extent,
-                  "Offset must not exceed Extent");
-    static_assert(Extent == dynamic_extent || Count == dynamic_extent ||
-                      Count <= Extent - Offset,
+    static_assert(Offset <= Extent, "Offset must not exceed Extent");
+    static_assert(Count == dynamic_extent || Count <= Extent - Offset,
                   "Count must not exceed Extent - Offset");
     CHECK(Extent != dynamic_extent || Offset <= size());
     CHECK(Extent != dynamic_extent || Count == dynamic_extent ||