Add unreachable instruction to Subzero.

tests_lit/reader_tests/unreachable.ll

+; Test parsing unreachable instruction.
+
+; RUN: llvm-as < %s | pnacl-freeze -allow-local-symbol-tables \
+; RUN:              | %llvm2ice -notranslate -verbose=inst -build-on-read \
+; RUN:                -allow-pnacl-reader-error-recovery \
+; RUN:                -allow-local-symbol-tables \
+; RUN:              | FileCheck %s
+
+define internal i32 @divide(i32 %num, i32 %den) {
+entry:
+  %cmp = icmp ne i32 %den, 0
+  br i1 %cmp, label %return, label %abort
+
+abort:                                            ; preds = %entry
+  unreachable
+
+return:                                           ; preds = %entry
+  %div = sdiv i32 %num, %den
+  ret i32 %div
+}
+
+; CHECK:      define internal i32 @divide(i32 %num, i32 %den) {
+; CHECK-NEXT: entry:
+; CHECK-NEXT:   %cmp = icmp ne i32 %den, 0
+; CHECK-NEXT:   br i1 %cmp, label %return, label %abort
+; CHECK-NEXT: abort:
+; CHECK-NEXT:   unreachable
+; CHECK-NEXT: return:
+; CHECK-NEXT:   %div = sdiv i32 %num, %den
+; CHECK-NEXT:   ret i32 %div
+; CHECK-NEXT: }