LoopVectorizer: Use matcher from PatternMatch.h for the min/max patterns

Also make some static function class functions to avoid having to mention the
class namespace for enums all the time.

No functionality change intended.

git-svn-id: https://llvm.org/svn/llvm-project/llvm/trunk@179886 91177308-0d34-0410-b5e6-96231b3b80d8