As I mention in the PR...

What's happening here is that the ipa_polymorphic_call_context 
constructor is calling walk_ssa_copies on a PHI node that has no 
arguments.  This happens because finalize_jump_threads eventually 
removes some PHI arguments as it's redirecting some edges, leaving a PHI 
with no arguments:

SR.33_23 = PHI <>

This should get cleaned up later, but the IPA polymorphic code gets 
called during the actual CFG clean-up, and walk_ssa_copies cannot handle 
an empty PHI.

Approved by Honza.

Fully tested on x86-64 Linux and verified that the patch fixes the ICE 
on an x86-64 Linux cross aarch64-linux-gnu cc1plus.

Committed to mainline.