Add support for lowering 32-bit/64-bit pointers

Summary:
This follows a previous patch that changes the X86 datalayout to represent
mixed size pointers (32-bit sext, 32-bit zext, and 64-bit) with address spaces
(https://reviews.llvm.org/D64931)

This patch implements the address space cast lowering to the corresponding
sign extension, zero extension, or truncate instructions.

Related to https://bugs.llvm.org/show_bug.cgi?id=42359

Reviewers: rnk, craig.topper, RKSimon

Subscribers: hiraditya, llvm-commits

Tags: #llvm

Differential Revision: https://reviews.llvm.org/D69639