[Codegen][X86] `LowerBUILD_VECTOR()`: improve lowering w/ multiple FREEZE-UNDEF ops

While we have great handling for UNDEF operands,
FREEZE-UNDEF operands are effectively normal operands.

We are better off "interleaving" such BUILD_VECTORS into a blend
between a splat of FREEZE-UNDEF, and "thawed" source BUILD_VECTOR,
both of which are more natural for us to handle.

Refs. https://github.com/llvm/llvm-project/commit/f738ab9075f838dd4365adf3a92ca1acced114d7#r95017306