[llvm] Refactor LaneBitmask to be a Bitset (PR #191757)

[Matt Arsenault via llvm-commits]

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@@ -194,8 +201,86 @@ template <unsigned NumBits> class Bitset {
     }
     return false;
   }
+
+  constexpr Bitset &operator<<=(unsigned N) {
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arsenm wrote:

It always was 

https://github.com/llvm/llvm-project/pull/191757