[X86] In lowerVectorShuffleAsBroadcast, make peeking through CONCAT_VECTORS work correctly if we already walked through a bitcast that changed the element size.
The CONCAT_VECTORS case was using the original mask element count to determine how to adjust the broadcast index. But if we looked through a bitcast the original mask size doesn't tell us anything about the concat_vectors.
This patch switchs to using the concat_vectors input element count directly instead.
Differential Revision: https://reviews.llvm.org/D53823
git-svn-id: https://llvm.org/svn/llvm-project/llvm/trunk@345626
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