cmd/compile: compile taking a long time for large computed struct literals

[mpvl]

Please answer these questions before submitting your issue. Thanks!

1. What version of Go are you using (go version)?
   tip
2. What operating system and processor architecture are you using (go env)?
   GOARCH="amd64"
   GOBIN=""
   GOEXE=""
   GOHOSTARCH="amd64"
   GOHOSTOS="darwin"
   GOOS="darwin"
   GORACE=""
   CC="clang"
   GOGCCFLAGS="-fPIC -m64 -pthread -fno-caret-diagnostics -Qunused-arguments -fmessage-length=0 -gno-record-gcc-switches -fno-common"
   CXX="clang++"
   CGO_ENABLED="1"
3. What did you do?
   Run the attached file.
   main.go.txt
4. What did you expect to see?
   Sub-second compile times.
5. What did you see instead?
   16s+ compile times.

[ianlancetaylor]

CC @randall77 @josharian

[randall77]

@dr2chase

[dr2chase]

I tried the faster-phi-location patch (since I had it immediately at hand) and it was no help at all.
Profile says that 70% of the time is in regalloc.

[gopherbot]

CL https://golang.org/cl/30163 mentions this issue.

[gopherbot]

CL https://golang.org/cl/30354 mentions this issue.