Corrected the descriptions of the llvm.writeport and llvm.writeio

Corrected the descriptions of the llvm.writeport and llvm.writeio
intrinsics.

Modified llvm.readio and llvm.writeio to use pointers to memory instead
of integers.  This should take care of problems such as different pointer
sizes, casting integers to pointers, weird architectural pointer types, etc.

Re-worded the description of llvm.readio and llvm.writeio so that it should
be more clear as to why they should be used over regular loads/stores for
I/O.

git-svn-id: https://llvm.org/svn/llvm-project/llvm/trunk@12867 91177308-0d34-0410-b5e6-96231b3b80d8