Remove dead elements from the MemorySet in load elimination.

Remove dead elements from the MemorySet in load elimination.

The old code killed bindings by storing `null`. It is possible to
construct straight-line code with takes time and space O(N^2) to do
load elimination, since all the useless nulled-out bindings are
re-walked on every killAffectedBy call.

Luckily most of our generated code has some joins which cause the
intersection to be computed which deletes the unused bindings.

BUG=
R=sigmund@google.com

Committed: https://github.com/dart-lang/sdk/commit/8d62ce1042e5a4e395701afeb92c11431d4c8b80

[sra1, 2017-03-22 22:13:30]

sra@google.com changed reviewers:
+ sigmund@google.com

[Siggi Cherem (dart-lang), 2017-03-23 19:29:17]

lgtm

[sra1, 2017-04-03 19:12:45]

Description was changed from

==========
Remove dead elements from the MemorySet in load elimination.

The old code killed bindings by storing `null`. It is possible to
construct straight-line code with takes time and space O(N^2) to do
load elimination, since all the useless nulled-out bindings are
re-walked on every killAffectedBy call.

Luckily most of our generated code has some joins which cause the
intersection to be computed which deletes the unused bindings.

BUG=
==========

to

==========
Remove dead elements from the MemorySet in load elimination.

The old code killed bindings by storing `null`. It is possible to
construct straight-line code with takes time and space O(N^2) to do
load elimination, since all the useless nulled-out bindings are
re-walked on every killAffectedBy call.

Luckily most of our generated code has some joins which cause the
intersection to be computed which deletes the unused bindings.

BUG=
R=sigmund@google.com

Committed:
https://github.com/dart-lang/sdk/commit/8d62ce1042e5a4e395701afeb92c11431d4c8b80
==========

[sra1, 2017-04-03 19:12:45]

Committed patchset #2 (id:20001) manually as
8d62ce1042e5a4e395701afeb92c11431d4c8b80 (presubmit successful).