[llvm] [InstCombine] Optimize icmp(sub(a, c), sub(b, c)) to icmp(a, b) if a, b, and c are pointers (PR #161698)

[Ryan Buchner via llvm-commits]

================
@@ -0,0 +1,109 @@
+; NOTE: Assertions have been autogenerated by utils/update_test_checks.py
+; RUN: opt < %s -passes=instcombine -S | FileCheck %s
+
+define ptr @icmp_ptrdiff_gt(ptr %0, ptr %1, ptr %2) {
----------------
bababuck wrote:

Using NSW runs into the issue that it the optimization would occur without adjusting the signed comparison into an unsigned comparison, and the pointer comparison won't behave correctly as a signed comparison. (I tried that approach initially).

https://github.com/llvm/llvm-project/pull/161698