[AMDGPU] Add Relocation Constant Support
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 <firstname.lastname@example.org>
Reviewers: tpr, nhaehnle
Reviewed By: nhaehnle
Subscribers: arsenm, kzhuravl, jvesely, wdng, yaxunl, dstuttard, t-tye, hiraditya, kerbowa, llvm-commits
Differential Revision: https://reviews.llvm.org/D76440