Make BitMask::flip also work w/o SSE2

Thanks miri!

Author: jonhoo

src/raw/bitmask.rs

@@ -32,7 +32,8 @@ impl BitMask {
     #[allow(clippy::cast_ptr_alignment)]
     #[cfg(feature = "raw")]
     pub unsafe fn flip(&mut self, index: usize) -> bool {
-        let mask = 1 << (index * BITMASK_STRIDE);
+        // NOTE: The + BITMASK_STRIDE - 1 is to set the high bit.
+        let mask = 1 << (index * BITMASK_STRIDE + BITMASK_STRIDE - 1);
         self.0 ^= mask;
         // The bit was set if the bit is now 0.
         self.0 & mask == 0