[mlir] NFC: untangle SCF Patterns.h and Transforms.h

These two headers both contained a strange mix of definitions related to
both patterns and non-pattern transforms. Put patterns and "populate"
functions into Patterns.h and standalone transforms into Transforms.h.

Depends On: D155223

Reviewed By: nicolasvasilache

Differential Revision: https://reviews.llvm.org/D155454