This commit moves code around: The helper functions/classes are moved
into `MemRefToLLVM.cpp`. This simplifies the code a bit: fewer
templatized functions, fewer function calls, fewer lines of code.
This commit also moves checks in `matchAndRewrite` to the beginning of
the functions, such that patterns bail out (`return failure()`) before
starting to modify any IR. (Apart from that, this change is NFC.) This
is in preparation of the One-Shot Dialect Conversion refactoring, which
will disallow pattern rollbacks.
bb21a6819b