Peephole optimize x == false and x != true

[syvb]

This adds peephole optimizations to make x == false, false == x, x != true, and true != x get optimized to !x in the instcombine MIR pass. That pass currently handles x == true -> x already.

[rust-highfive]

r? @petrochenkov

(rust-highfive has picked a reviewer for you, use r? to override)

[wesleywiser]

@bors try @rust-timer queue

[rust-timer]

Awaiting bors try build completion.

@rustbot label: +S-waiting-on-perf

[bors]

⌛ Trying commit 2f1c219 with merge e2c32a71d8e2919c91fcde05b55ff8c486a9cdc2...

[petrochenkov]

r? @wesleywiser

[bors]

☀️ Try build successful - checks-actions
Build commit: e2c32a71d8e2919c91fcde05b55ff8c486a9cdc2 (e2c32a71d8e2919c91fcde05b55ff8c486a9cdc2)

[rust-timer]

Queued e2c32a71d8e2919c91fcde05b55ff8c486a9cdc2 with parent e4a6032, future comparison URL.

[rust-timer]

Finished benchmarking try commit (e2c32a71d8e2919c91fcde05b55ff8c486a9cdc2): comparison url.

Benchmarking this pull request likely means that it is perf-sensitive, so we're automatically marking it as not fit for rolling up. Please note that if the perf results are neutral, you should likely undo the rollup=never given below by specifying rollup- to bors.

Importantly, though, if the results of this run are non-neutral do not roll this PR up -- it will mask other regressions or improvements in the roll up.

@bors rollup=never
@rustbot label: +S-waiting-on-review -S-waiting-on-perf

[wesleywiser]

Looks great!

[wesleywiser]

@bors r+

[bors]

📌 Commit 2f1c219 has been approved by wesleywiser

[bors]

⌛ Testing commit 2f1c219 with merge d45d205...

[bors]

☀️ Test successful - checks-actions
Approved by: wesleywiser
Pushing d45d205 to master...