Scalarizer for masked.gather and masked.scatter intrinsics.

Scalarizer for masked.gather and masked.scatter intrinsics.
When the target does not support these intrinsics they should be converted to a chain of scalar load or store operations.
If the mask is not constant, the scalarizer will build a chain of conditional basic blocks.
I added isLegalMaskedGather() isLegalMaskedScatter() APIs.

Differential Revision: http://reviews.llvm.org/D13722

git-svn-id: https://llvm.org/svn/llvm-project/llvm/trunk@251237 91177308-0d34-0410-b5e6-96231b3b80d8