[SDAG] Add new AssertAlign ISD node.

[SDAG] Add new AssertAlign ISD node.

Summary:
- AssertAlign node records the guaranteed alignment on its source node,
  where these alignments are retrieved from alignment attributes in LLVM
  IR. These tracked alignments could help DAG combining and lowering
  generating efficient code.
- In this patch, the basic support of AssertAlign node is added. So far,
  we only generate AssertAlign nodes on return values from intrinsic
  calls.
- Addressing selection in AMDGPU is revised accordingly to capture the
  new (base + offset) patterns.

Reviewers: arsenm, bogner

Subscribers: jvesely, wdng, nhaehnle, tpr, hiraditya, kerbowa, llvm-commits

Tags: #llvm

Differential Revision: https://reviews.llvm.org/D81711