[m-rev.] for post-commit review: reduce overheads in recompute_instmap_delta_1

[Zoltan Somogyi]

This gets rid of some overhead I noticed while fixing Mantis bug #512.
For review by anyone.

Zoltan.
-------------- next part --------------
A non-text attachment was scrubbed...
Name: Log.rid
Type: application/octet-stream
Size: 555 bytes
Desc: not available
URL: <http://lists.mercurylang.org/archives/reviews/attachments/20201023/531005d4/attachment-0002.obj>
-------------- next part --------------
A non-text attachment was scrubbed...
Name: DIFF.rid
Type: application/octet-stream
Size: 18129 bytes
Desc: not available
URL: <http://lists.mercurylang.org/archives/reviews/attachments/20201023/531005d4/attachment-0003.obj>