Subzero: Prune unreachable nodes after constructing the Cfg.

tests_lit/llvm2ice_tests/prune_unreachable.ll

Index: tests_lit/llvm2ice_tests/prune_unreachable.ll
diff --git a/tests_lit/llvm2ice_tests/prune_unreachable.ll b/tests_lit/llvm2ice_tests/prune_unreachable.ll
new file mode 100644
index 0000000000000000000000000000000000000000..0c77acd1a8dfeb41332fe33505cd52bfe5f321f5
--- /dev/null
+++ b/tests_lit/llvm2ice_tests/prune_unreachable.ll
@@ -0,0 +1,22 @@
+; This tests that unreachable basic blocks are pruned from the CFG, so that
+; liveness analysis doesn't detect inconsistencies.
+
+; RUN: %p2i -i %s --filetype=obj --disassemble --args -Om1 | FileCheck %s
+; RUN: %p2i -i %s --filetype=obj --disassemble --args -O2 | FileCheck %s
+
+declare void @abort()
+
+define i32 @unreachable_block() {
+entry:
+  ; ret_val has no reaching uses and so its assignment may be
+  ; dead-code eliminated.
+  %ret_val = add i32 undef, undef
+  call void @abort()
+  unreachable
+label:
+  ; ret_val has no reaching definitions, causing an inconsistency in
+  ; liveness analysis.
+  ret i32 %ret_val
+}
+
+; CHECK-LABEL: unreachable_block