Add support for pointer types in patterns

Summary:
This adds support for defining patterns for global isel using pointer
types, for example:

def : Pat<(load GPR32:$src),
          (p1 (LOAD GPR32:$src))>;

DAGISelEmitter will ignore the pointer information and treat these
types as integers with the same bit-width as the pointer type.

Reviewers: dsanders, rtereshin, arsenm

Reviewed By: arsenm

Subscribers: Petar.Avramovic, wdng, rovka, kristof.beyls, jfb, volkan, llvm-commits

Tags: #llvm

Differential Revision: https://reviews.llvm.org/D57065

llvm-svn: 354510