AMDGPU/GlobalISel: Remove unnecessary legality constraint for G_EXTRACT

Summary:
We were marking G_EXTRACT operations unsupported if the output type
was larger than the input type.  I don't see how this could ever actually
happen, so I dropped the constraint.  Doing this makes it possible to
reuse the same legality code for G_INSERT.

Reviewers: arsenm

Reviewed By: arsenm

Subscribers: kzhuravl, wdng, nhaehnle, yaxunl, rovka, kristof.beyls, dstuttard, tpr, t-tye, llvm-commits

Differential Revision: https://reviews.llvm.org/D49600

llvm-svn: 337794