[AMDGPU] Add Relocation Constant Support

Summary:
This change adds amdgcn.reloc.constant intrinsic to the amdgpu backend, which will compile into a relocation entry in the resulting elf.

The intrinsics takes a MetadataNode (String) as its only argument, which specifies the symbol name of the relocation entry.

`SelectionDAGBuilder::getValueImpl` is changed to allow metadata operands passed through to ISel.

Author: csyonghe <yonghe@google.com>

Reviewers: tpr, nhaehnle

Reviewed By: nhaehnle

Subscribers: arsenm, kzhuravl, jvesely, wdng, yaxunl, dstuttard, t-tye, hiraditya, kerbowa, llvm-commits

Tags: #llvm

Differential Revision: https://reviews.llvm.org/D76440