[X86][AVX512] Fold extract_element(bitcast(<X x i1>) -> bitcast(extract_subvector())

On AVX512, extract legal bool vectors as bool subvectors before bitcasting to scalars to avoid spilling to stack.

This helps rust which internally represents bool vectors as bool arrays

It also exposes more missed opportunities to use the KADD instruction to add masks together before moving to gpr

Fixes #58546