[X86][AVX] lowerShuffleAsBroadcast - MOVDDUP(SCALAR_TO_VECTOR(X)) -> BROADCAST(X)

Prefer broadcast from scalar on AVX targets as this makes it easier for later folds to strip away bitcasts etc.

This helps a lot with the AVX1 poor codegen from PR49658.

There's a trivial regression in bitcast-int-to-vector-bool-*ext.ll tests due to SimplifyDemandedBits not being able to see a multi-use case, but there's bigger existing codegen issues to be addressed first in those tests (unnecessary NOTs).