[dev.regabi] all: merge master into dev.regabi

The list of conflicted files for this merge is:

   src/cmd/compile/internal/gc/inl.go
   src/cmd/compile/internal/gc/order.go
   src/cmd/compile/internal/gc/ssa.go
   test/fixedbugs/issue20415.go
   test/fixedbugs/issue22822.go
   test/fixedbugs/issue28079b.go

inl.go was updated for changes on dev.regabi: namely that OSELRECV has
been removed, and that OSELRECV2 now only uses List, rather than both
Left and List.

order.go was updated IsAutoTmp is now a standalone function, rather
than a method on Node.

ssa.go was similarly updated for new APIs involving package ir.

The tests are all merging upstream additions for gccgo error messages
with changes to cmd/compile's error messages on the dev.regabi branch.

Change-Id: Icaaf186d69da791b5994dbb6688ec989caabec42