cmd/compile: SSA code generation bug

[dgryski]

A TravisCI test failure for github.com/go-sql-driver/mysql ( https://travis-ci.org/go-sql-driver/mysql/builds/114356883 ) caught this SSA code generation failure.

Code and test extracted here: https://play.golang.org/p/YPy3UA4X7M

Reproduced on my local machine:

<dgryski@kamek[w] \ʕ◔ϖ◔ʔ/ > go version
go version go1.6 linux/amd64
<dgryski@kamek[w] \ʕ◔ϖ◔ʔ/ > go run main.go
<dgryski@kamek[w] \ʕ◔ϖ◔ʔ/ > ~/work/src/cvs/go.tip/bin/go version
go version devel +133c26b Mon Mar 7 20:53:49 2016 +0000 linux/amd64
<dgryski@kamek[w] \ʕ◔ϖ◔ʔ/ > ~/work/src/cvs/go.tip/bin/go run main.go
2016/03/07 23:02:47 fe0000000100000000: expected 16777216, got 4294967550
2016/03/07 23:02:47 4294967550: expected fe0000000100000000, got fefe00000001000000
2016/03/07 23:02:47 fef0debc9a78563412: expected 1311768467463790320, got 3771334343958393086
2016/03/07 23:02:47 3771334343958393086: expected fef0debc9a78563412, got fefef0debc9a785634
2016/03/07 23:02:47 feffffffffffffffff: expected 18446744073709551615, got 18446744073709551614
2016/03/07 23:02:47 18446744073709551614: expected feffffffffffffffff, got fefeffffffffffffff

[dgryski]

A git bisect points at 12e6045

[gopherbot]

CL https://golang.org/cl/20333 mentions this issue.