[flang] still apply vectorization cost model with IVDEP (#180760)

The current implementation of `!DIR IVDEP` leads flang to bypass LLVM
cost model and always vectorize the loop carrying `!DIR$ IVDEP`.

IVDEP is an extension and its documentation varies, and while it usually
leads to vectorization because it is added on loops where it is usually
profitable, its documentation only tells it is meant to tell the
compiler that there are no loop carried dependencies and that the loop
is safe to vectorize.

In some application, such directive may have been added to help the
compiler proving it is safe to vectorize, but vectorizing is not always
the best choice for all architectures. The cost model should still be
applied. This is at least the case for classic flang.

When users want vectorization to happen, they should use `!DIR$ VECTOR
ALWAYS`.

This patch updates flang to not emit `llvm.loop.vectorize.enable` just
because IVDEP was seen. Instead, IVDEP now only controls the emissions
of the access groups to translate the independence of the accesses and
leave the vectorization decision up to the cost model. `!DIR$ VECTOR
ALWAYS` can be used in combination with IVDEP to force vectorization (it
causes the emission of `llvm.loop.vectorize.enable`).