Quick: Abolish kMirOpCheckPart2.

The tricks played with kMirOpCheckPart2 are making the
native GC map generation unnecessarily complex. They have
caused problems in the past and now there is bad interaction
with the DCE. Rather than fixing it time and again, remove
the pseudo-insn.

(The whole purpose of those tricks seems to be to allow the
register tracking to be used for the throwing insn before
resetting the tracking for the next block. However, it's
questionable whether that's better than processing the
throwing insn with the subsequent instructions.)

Bug: 20736048

(cherry picked from commit e299f167)

Change-Id: I8a60d26c5e6b6b608d68b8bb6b66d411f9a28f90