Cleanup comparisons to VariableArrayType::Static for non-VLAs

The enum is part of ArrayType, so there is no functional change, but comparing
to ArrayType::Static for non-VLAs makes more sense.

git-svn-id: https://llvm.org/svn/llvm-project/cfe/trunk@213446 91177308-0d34-0410-b5e6-96231b3b80d8