darwin/386 is no longer supported by Go.

(cherry picked from commit 3c24ef8)

Mentioning the flag on every run that produces diagnostics has turned
out to be too annoying. Most users will remember that the flag exists
after the first time we've told them about it. After that, we're just
making output longer and more noisy for people. This is especially
evident when run from inside editors.

Instead of adding more heuristics or flags to control this behavior,
remove it. We'll have to figure out a better way of teaching our users.

This reverts 9cc924a,
9b21780 and 08a27f4.

(cherry picked from commit 731fdc8)

Closes: gh-892 [via git-merge-pr]
(cherry picked from commit b4fb836)

When we added sigma nodes, a bunch of checks stopped working in
branches, because we naively type-asserted to our desired IR types,
without looking through sigma nodes.

Closes gh-855

(cherry picked from commit 5047034)

See 5047034.

(cherry picked from commit ce187b4)

In commit ae60410 we changed
extractConsts to not collect all distinct constants from phi nodes. We
didn't notice that the function no longer returned more than one
constant and didn't need to return a slice anymore.

In this change, we change the signature and name, and replace all
looping with a single if != nil check.

Additionally, we introduce the extractConstExpectKind helper, which
takes care of checking that the constant is of the right kind, and not
untyped nil. This helper removes a lot of boilerplate.

(cherry picked from commit 1735532)

Closes gh-898

(cherry picked from commit da78760)

(cherry picked from commit c69da86)

This addresses https://blog.golang.org/path-security.

(cherry picked from commit f563184)