[dev.typeparams] all: merge dev.regabi (063c72f0) into dev.typeparams

Eager re-sync-branch to keep Git history reasonably accurate, since
Git lacks a better way of encoding partial merges like CL 286172.

Conflicts:

- src/cmd/compile/internal/inline/inl.go
- src/cmd/compile/internal/noder/import.go
- src/cmd/compile/internal/noder/noder.go

Merge List:

+ 2021-01-25 063c72f0 [dev.regabi] cmd/compile: backport changes from dev.typeparams (9456804e)
+ 2021-01-23 d05d6fab [dev.regabi] cmd/compile: replace ir.Name map with ir.NameSet for SSA 2
+ 2021-01-23 48badc5f [dev.regabi] cmd/compile: fix escape analysis problem with closures
+ 2021-01-23 51e1819a [dev.regabi] cmd/compile: scan body of closure in tooHairy to check for disallowed nodes

Change-Id: I48c0435f7aaf56f4aec26518a7459e9d95a51e9c