AMDGPU should report 64-bit add/sub with overflow as legal

[arsenm]

Currently for SelectionDAG, we lie and report 64-bit add and sub as legal. For the same reason, we should report the with-overflow cases as legal. The 64-bit pseudo expansion does produce the carry-out, so it should be simple to wire the overflow ops to use it.

This will enable various combines that check the legality of the overflow operation. One example is the combine added by #161651; this should be profitable for the i64 case.

[llvmbot]

@llvm/issue-subscribers-backend-amdgpu

Author: Matt Arsenault (arsenm)

Currently for SelectionDAG, we lie and report 64-bit add and sub as legal. For the same reason, we should report the 64-bit cases as legal. The 64-bit pseudo expansion does produce the carry-out, so it should be simple to wire the overflow ops to use it.

This will enable various combines that check the legality of the overflow operation. One example is the combine added by #161651; this should be profitable for the i64 case.