dart2js: eliminate useless conditionals

dart2js: eliminate useless conditionals

1. Simplify HPhi nodes that merge refinements of the same value.

2. Replace the condition of HIf with a constant when the HIf does nothing (no controlled statements or phis).

Together with existing optimizations, this can optimize `s?.toString()` to `s` when `s` is known to be a nullable String.

BUG=
R=efortuna@google.com

Committed: https://github.com/dart-lang/sdk/commit/b0599b264736a3dff5b9ea26de47deac427b5447

[sra1, 2017-04-14 17:29:12]

Patchset #3 (id:40001) has been deleted

[sra1, 2017-04-14 17:29:18]

Patchset #2 (id:20001) has been deleted

[sra1, 2017-04-14 18:09:41]

Description was changed from

==========
dart2js: simply phis in dead code elimination

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

to

==========
dart2js: eliminate useless conditionals

1. Simplify HPhi nodes that merge refinements of the same value.

2. Replace the condition of HIf with a constantg when the HIf does nothing.

Together with existing optimizations, this can optimize `s?.toString()` to `s`
when `s` is known to be a nullable String.

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

[sra1, 2017-04-14 18:09:41]

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

[sra1, 2017-04-14 18:45:11]

Patchset #3 (id:80001) has been deleted

[sra1, 2017-04-14 18:45:19]

Patchset #1 (id:1) has been deleted

[sra1, 2017-04-14 18:45:25]

Patchset #1 (id:60001) has been deleted

[sra1, 2017-04-14 18:48:35]

Patchset #1 (id:100001) has been deleted

[sra1, 2017-04-14 18:51:23]

Description was changed from

==========
dart2js: eliminate useless conditionals

1. Simplify HPhi nodes that merge refinements of the same value.

2. Replace the condition of HIf with a constantg when the HIf does nothing.

Together with existing optimizations, this can optimize `s?.toString()` to `s`
when `s` is known to be a nullable String.

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

to

==========
dart2js: eliminate useless conditionals

1. Simplify HPhi nodes that merge refinements of the same value.

2. Replace the condition of HIf with a constant when the HIf does nothing (no
controlled statements or phis).

Together with existing optimizations, this can optimize `s?.toString()` to `s`
when `s` is known to be a nullable String.

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

[Emily Fortuna, 2017-04-14 20:11:45]

lgtm

[sra1, 2017-04-14 21:34:17]

Description was changed from

==========
dart2js: eliminate useless conditionals

1. Simplify HPhi nodes that merge refinements of the same value.

2. Replace the condition of HIf with a constant when the HIf does nothing (no
controlled statements or phis).

Together with existing optimizations, this can optimize `s?.toString()` to `s`
when `s` is known to be a nullable String.

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

to

==========
dart2js: eliminate useless conditionals

1. Simplify HPhi nodes that merge refinements of the same value.

2. Replace the condition of HIf with a constant when the HIf does nothing (no
controlled statements or phis).

Together with existing optimizations, this can optimize `s?.toString()` to `s`
when `s` is known to be a nullable String.

BUG=
R=efortuna@google.com

Committed:
https://github.com/dart-lang/sdk/commit/b0599b264736a3dff5b9ea26de47deac427b5447
==========

[sra1, 2017-04-14 21:34:18]

Committed patchset #1 (id:120001) manually as
b0599b264736a3dff5b9ea26de47deac427b5447 (presubmit successful).