[llvm] [KnownBits] Add operator<<=(unsigned) and operator>>=(unsigned). NFC (PR #155751)

[Jay Foad via llvm-commits]

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@@ -487,6 +487,20 @@ struct KnownBits {
   /// Update known bits based on XORing with RHS.
   LLVM_ABI KnownBits &operator^=(const KnownBits &RHS);
 
+  /// Shift known bits left by ShAmt. Shift in bits are unknown.
+  KnownBits &operator<<=(unsigned ShAmt) {
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jayfoad wrote:

This breaks the convention that `KnownBits::operatorXX` corresponds to doing `operatorXX` on the underlying value -- if we followed that convention then this operator would shift in known zeros.

https://github.com/llvm/llvm-project/pull/155751