[SCEV] Move some struct declarations inside functions; NFC

Reduces the scope over which the struct is visible, making its usages
obvious.  I did not move structs in cases where this wasn't a clear
win (the struct is too large, or is grouped in some other interesting
way).

git-svn-id: https://llvm.org/svn/llvm-project/llvm/trunk@255003 91177308-0d34-0410-b5e6-96231b3b80d8