[llvm] [RegisterCoalescer] Fix LiveRange::overlaps assert on empty interval (PR #194771)

[Matt Arsenault via llvm-commits]

================
@@ -0,0 +1,39 @@
+# RUN: llc -mtriple=x86_64-unknown-linux-gnu -run-pass=register-coalescer -o - %s | FileCheck %s
+
+# This test demonstrates a bug in RegisterCoalescer where LiveRange::overlaps()
+# is called on a register with an empty live interval, leading to a crash. 
+# Overlapping check must be symmetric.
+#
+# The pattern needed to reproduce the crash:
+#   1. Copy A: %dst = COPY %src, where %dst is terminal (no other copy affinity)
+#   2. Copy B: %src = COPY %undef_reg, redefining %src from an undefined register
+#   3. %undef_reg has no definition (empty live interval)
+#   4. %undef_reg is non-terminal (appears in at least two copies)
+#
+
+# CHECK-LABEL: name: empty_interval_overlaps_crash
----------------
arsenm wrote:

Use update_mir_test_checks 

https://github.com/llvm/llvm-project/pull/194771