Add regression test for null check elimination.

Prompted by
    https://android-review.googlesource.com/110090

Bug: 17969907
Change-Id: I938c27cda0681b9431d69baf4eafa7ca2f9b5c9c
diff --git a/compiler/dex/mir_optimization.cc b/compiler/dex/mir_optimization.cc
index 2ac0b51..84c056d 100644
--- a/compiler/dex/mir_optimization.cc
+++ b/compiler/dex/mir_optimization.cc
@@ -929,6 +929,8 @@
         mir->optimization_flags |= MIR_IGNORE_NULL_CHECK;
       } else {
         // Do the null check.
+        // TODO: Rewrite the pass to converge first before doing any modifications so that
+        // we don't lose the MIR_IGNORE_NULL_CHECK here if previously set by some other pass.
         mir->optimization_flags &= ~MIR_IGNORE_NULL_CHECK;
         // Mark s_reg as null-checked
         ssa_regs_to_check->ClearBit(src_sreg);