Subzero: Control whether deleted instructions are retained.

Subzero: Control whether deleted instructions are retained.

Normally, deleted instructions are preserved in the Cfg, and printed as part of dump output.  This helps debugging by partially explaining the provenance of new instructions that originated from the deleted instructions.

However, these instructions slow down iteration over the instruction list, and checking their deleted status may pollute the cache.

As such, in a non-DUMP enabled build, we repurpose the renumberInstructions() pass to also unlink deleted instructions as needed.

A flag is provided to override this behavior, in case we have to debug a situation where a bug only manifests in a DUMP build and not a non-DUMP build, or vice versa.

BUG= https://bugs.chromium.org/p/nativeclient/issues/detail?id=4360
R=eholk@chromium.org, jpp@chromium.org

Review URL: https://codereview.chromium.org/1790063003 .