[ScalarizeMaskedMemIntrinsic] Move from CodeGen into Transforms

ScalarizeMaskedMemIntrinsic is currently a codeGen level pass. The pass
is actually operating on IR level and does not use any code gen specific
passes.  It is useful to move it into transforms directory so that it
can be more widely used as a mid-level transform as well (apart from
usage in codegen pipeline).
In particular, we have a usecase downstream where we would like to use
this pass in our mid-level pipeline which operates on IR level.

The next change will be to add support for new PM.

Reviewers: craig.topper, apilipenko, skatkov
Reviewed-By: skatkov
Differential Revision: https://reviews.llvm.org/D92407