[X86] getBT - attempt to peek through aext(and(trunc(x),c)) mask/modulo

Ideally we'd fold this with generic DAGCombiner, but that only works for !isTruncateFree cases - we might be able to adapt IsDesirableToPromoteOp to find truncated src ops in the future, but for now just use this peephole.

Noticed in Issue #55138