[IR] Add samesign flag to icmp instruction

[elhewaty]

As suggested here we will add the samesign flag to the icmp instruction. We can split this into separate tasks:

• Support the samesign flag in the IR (Tokenizer, Parser, as a keyword, ...etc)
• Update the SCEVExpander.

#111419 may handle the previous two goals.

• Propagate the flag to the SelectionDAG. [SelectionDAG] Add preliminary plumbing for samesign flag #112354
• Handle the flag in GlobalISel.
• Teach llvm-reduce how to strip this. [llvm-reduce] Reduce samesign flag from icmp #115492

I will try to work on these tasks under the guidance of the community if it works for you.

CC @nikic @arsenm @tschuett @dtcxzyw @topperc

[tschuett]

#114267

[nikic]

Closing this as the initial bring-up work is complete, the rest is optimization/analysis support.