[X86] Simplify LowerBITCAST code for v2i32/v4i16/v8i8/i64->mmx/i64/f64 bitcast.

Previously this code generated its own extracts and build_vector. But we can use a simpler concat_vectors or scalar_to_vector operation and let type legalization do additional legalization of those operations.

llvm-svn: 348087