Auto merge of #150 - andjo403:repeat, r=Amanieu

micro optimize repeat function

makes it easier for the compiler and the reader with less code :)

for "prof" https://godbolt.org/z/5bdD2b
when I tried cargo bench I got very large variance between runs was multiple test that one time had 2x improvement to the next time have a minor regression so have more trust in the llvm-mca result.

src/raw/generic.rs

@@ -27,11 +27,7 @@ pub const BITMASK_MASK: BitMaskWord = 0x8080_8080_8080_8080_u64 as GroupWord;
 /// Helper function to replicate a byte across a `GroupWord`.
 #[inline]
 fn repeat(byte: u8) -> GroupWord {
-    let repeat = GroupWord::from(byte);
-    let repeat = repeat | repeat.wrapping_shl(8);
-    let repeat = repeat | repeat.wrapping_shl(16);
-    // This last line is a no-op with a 32-bit GroupWord
-    repeat | repeat.wrapping_shl(32)
+    GroupWord::from_ne_bytes([byte; Group::WIDTH])
 }
 
 /// Abstraction over a group of control bytes which can be scanned in