[X86] LowerTRUNCATE - avoid creating extract_subvector(bitcast(vec)) patterns

We have a generic DAG combine to attempt to fold extract_subvector(bitcast(vec)) -> bitcast(extract_subvector(vec)) but if we create these patterns late in lowering then we often miss them.

Noticed while investigating Issue #55648 which gets caught in an infinite loop trying to split extract_subvector(bitcast(vselect()) patterns - this doesn't fix the issue yet but reduces the regressions from the WIP fix.