[X86][AVX] lowerShuffleAsBroadcast - improve load folding by avoiding bitcasts

AVX1 broadcasts were failing as we were adding bitcasts that caused MayFoldLoad's hasOneUse to return false.

This patch stops introducing bitcasts so early and also replaces the broadcast index scaling through bitcasts (which can't succeed in some cases) to instead just keep track of the bitoffset which can be converted back to the broadcast index later on.

Differential Revision: https://reviews.llvm.org/D58888

llvm-svn: 356043