[ARM] GlobalISel: Lower pointer args and returns

[ARM] GlobalISel: Lower pointer args and returns

It is important to change the ArgInfo's type from pointer to integer, otherwise
the CC assign function won't know what to do. Instead of hacking it up, we use
ComputeValueVTs and introduce some of the helpers that we will need later on for
lowering more complex types.

git-svn-id: https://llvm.org/svn/llvm-project/llvm/trunk@293889 91177308-0d34-0410-b5e6-96231b3b80d8