improve performance of span_iterator w/ clang (#1166)

* improve performance of span_iterator w/ clang

Issue: #1165

Before this PR, the range-for loop was ~3300x slower. After this PR, it
is ~1.005x slower

The clang optimizer is very good at optimizing `current != end`, so
we changed to this idiom. This moves the Expects assertion into the
constructor instead of on the hot-path which is called whenever either
operator++ or operator* is called.

Note: The codegen for the assertion is still a missed optimization,
but less worrisome as it only happens once per iterator.

Note: benchmarks on M1 Macbook Pro w/ Apple Clang 16.0.0

include/gsl/span

@@ -140,7 +140,9 @@ namespace details
 
         constexpr span_iterator(pointer begin, pointer end, pointer current)
             : begin_(begin), end_(end), current_(current)
-        {}
+        {
+            Expects(begin_ <= current_ && current <= end_);
+        }
 
         constexpr operator span_iterator<const Type>() const noexcept
         {
@@ -149,21 +151,18 @@ namespace details
 
         constexpr reference operator*() const noexcept
         {
-            Expects(begin_ && end_);
+            Expects(current_ != end_);
-            Expects(begin_ <= current_ && current_ < end_);
             return *current_;
         }
 
         constexpr pointer operator->() const noexcept
         {
-            Expects(begin_ && end_);
+            Expects(current_ != end_);
-            Expects(begin_ <= current_ && current_ < end_);
             return current_;
         }
         constexpr span_iterator& operator++() noexcept
         {
-            Expects(begin_ && current_ && end_);
+            Expects(current_ != end_);
-            Expects(current_ < end_);
             // clang-format off
             GSL_SUPPRESS(bounds.1) // NO-FORMAT: attribute
             // clang-format on
@@ -180,8 +179,7 @@ namespace details
 
         constexpr span_iterator& operator--() noexcept
         {
-            Expects(begin_ && end_);
+            Expects(begin_ != current_);
-            Expects(begin_ < current_);
             --current_;
             return *this;
         }