[LV] Move isLegalMasked* functions from Legality to CostModel

All SIMD architectures can emulate masked load/store/gather/scatter
through element-wise condition check, scalar load/store, and
insert/extract. Therefore, bailing out of vectorization as legality
failure, when they return false, is incorrect. We should proceed to cost
model and determine profitability.

This patch is to address the vectorizer's architectural limitation
described above. As such, I tried to keep the cost model and
vectorize/don't-vectorize behavior nearly unchanged. Cost model tuning
should be done separately.

Please see
http://lists.llvm.org/pipermail/llvm-dev/2018-January/120164.html for
RFC and the discussions.

Closes D43208.

Patch by: Hideki Saito <hideki.saito@intel.com>

llvm-svn: 326079