[X86] In lowerVectorShuffleAsBroadcast, make peeking through CONCAT_VECTORS work...

[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

llvm-svn: 345626