[X86] Prefer isTypeLegal over checking isSimple in a DAG combine.

Simple types are a superset of what all in tree targets in LLVM could possibly have a legal type. This means the behavior of using isSimple to check for a supported type for X86 could change over time. For example, this could would change if a v256i1 type was added to MVT in the future.

llvm-svn: 343995