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

git-svn-id: 0ff597fd157e6f4fc38580e8d64ab130330d2411@343995 91177308-0d34-0410-b5e6-96231b3b80d8

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