Stay in vectors longer before doing a horizontal sum

Author: Catfish-Man

stdlib/public/core/StringUTF16View.swift

@@ -533,14 +533,18 @@ extension String.UTF16View {
       //Find the number of continuations (0b10xxxxxx)
       let sValue = Builtin.loadRaw(readPtr._rawValue) as S
       let continuations = S.zero.replacing(with: S.one, where: sValue .< -65 + 1)
-      let continuationCount = Int(continuations.wrappedSum())
 
       //Find the number of 4 byte code points (0b11110xxx)
       let uValue = Builtin.loadRaw(readPtr._rawValue) as U
-      let fourBytes = U.zero.replacing(with: U.one, where: uValue .>= 0b11110000)
-      let fourByteCount = Int(fourBytes.wrappedSum())
-            
-      utf16Count &+= (U.scalarCount - continuationCount) + fourByteCount
+      let fourBytes = S.zero.replacing(
+        with: S.one,
+        where: unsafeBitCast(
+          uValue .>= 0b11110000,
+          to: SIMDMask<S.MaskStorage>.self
+        )
+      )
+      
+      utf16Count &+= U.scalarCount + Int((fourBytes &- continuations).wrappedSum())
 
       readPtr += MemoryLayout<U>.stride
     }