AMDGPU/GlobalISel: Fix RegBankSelect for GEP.

This is basically a pointer typed add, so shouldn't be any different.
This was assuming everything was an SGPR, which is not true.

Also cleanup legality for GEP. I don't seem to be seeing the problem
the hack marking s64 as a legal pointer type the comment mentions.

git-svn-id: https://llvm.org/svn/llvm-project/llvm/trunk@354067 91177308-0d34-0410-b5e6-96231b3b80d8
3 files changed