[ValueTracking] Improve the coverage of isKnownToBeAPowerOfTwo for vscale

this PR tries to match the following pattern, seperate from D156881
     ```
     %vscale = call i64 @llvm.vscale.i64()
     %shift = shl nuw nsw i64 %vscale, 11
     ```
Now, we only check the shl recursively when the OrZero is true.

Reviewed By: goldstein.w.n
Differential Revision: https://reviews.llvm.org/D157062