Avoid some Unsafe.As (#118655)

Author: xtqqczze

src/libraries/System.Private.CoreLib/src/System/Memory.cs

@@ -284,7 +284,7 @@ public Span<T> Span
                     {
                         // Special-case string since it's the most common for ROM<char>.
 
-                        refToReturn = ref Unsafe.As<char, T>(ref Unsafe.As<string>(tmpObject).GetRawStringData());
+                        refToReturn = ref Unsafe.As<char, T>(ref ((string)tmpObject).GetRawStringData());
                         lengthOfUnderlyingSpan = Unsafe.As<string>(tmpObject).Length;
                     }
                     else if (RuntimeHelpers.ObjectHasComponentSize(tmpObject))

src/libraries/System.Private.CoreLib/src/System/ReadOnlyMemory.cs

@@ -206,7 +206,7 @@ public ReadOnlySpan<T> Span
                     {
                         // Special-case string since it's the most common for ROM<char>.
 
-                        refToReturn = ref Unsafe.As<char, T>(ref Unsafe.As<string>(tmpObject).GetRawStringData());
+                        refToReturn = ref Unsafe.As<char, T>(ref ((string)tmpObject).GetRawStringData());
                         lengthOfUnderlyingSpan = Unsafe.As<string>(tmpObject).Length;
                     }
                     else if (RuntimeHelpers.ObjectHasComponentSize(tmpObject))

src/libraries/System.Private.CoreLib/src/System/SearchValues/Strings/Helpers/AhoCorasickNode.cs

@@ -41,7 +41,7 @@ public readonly bool TryGetChild(char c, out int index)
 
             if (children.GetType() == typeof(int[]))
             {
-                int[] table = Unsafe.As<int[]>(children);
+                int[] table = (int[])children;
                 if (c < (uint)table.Length)
                 {
                     index = table[c];

src/libraries/System.Private.CoreLib/src/System/String.Manipulation.cs

@@ -924,7 +924,7 @@ private static string JoinCore<T>(ReadOnlySpan<char> separator, IEnumerable<T> v
             {
                 if (values.GetType() == typeof(List<string?>)) // avoid accidentally bypassing a derived type's reimplementation of IEnumerable<T>
                 {
-                    return JoinCore(separator, CollectionsMarshal.AsSpan(Unsafe.As<List<string?>>(values)));
+                    return JoinCore(separator, CollectionsMarshal.AsSpan((List<string?>)values));
                 }
 
                 if (values is string?[] valuesArray)