Avoid removing new-instance instruction twice.

Includes a fail-before/pass-after regression test.

Rationale:
In some corner cases, one new instance reached more than one
java.lang.String.<init> method call. As a result, the new
instance instruction appeared multiple times in the vector
uninitialized_strings_. A second removal crashes the compiler.
This change list avoid the crash by simply skipping instructions
that are already removed.

BUG=27847265

Change-Id: I7f8a4f84ea3c184e1529ec3e90bd6749c83c445b