[X86] Simplify the type legality checking for (FM)ADDSUB/SUBADD matching. NFCI

Rather than enumerating all specific types, for the DAG combine we can just use TLI::isTypeLegal and an SSE3 check. For the BUILD_VECTOR version we already know the type is legal so we just need to check SSE3.

llvm-svn: 327649