Rewrite null check elimination to work on dalvik regs.

And move the null check and class init check elimination
before the SSA transformation. The new pass ordering is in
anticipation of subsequent changes. (An improved class init
check elimination can benefit special method inlining. An
improved block combination pass before SSA transformation
can improve compilation time.)

Also add tests for the NCE.

Change-Id: Ie4fb1880e06334a703295aef454b437d58a3e878