cmd/compile: replace ir.Name map with ir.NameSet

[oiooj]

As CL 282212 mentioned, we should clean all map[*ir.Name]bool with ir.NameSet.

I'm working on this clean.

/cc @mdempsky

[gopherbot]

Change https://golang.org/cl/284897 mentions this issue: [dev.regabi] cmd/compile: replace ir.Name map with ir.NameSet in inlining

[gopherbot]

Change https://golang.org/cl/284898 mentions this issue: [dev.regabi] cmd/compile: replace ir.Name map with ir.NameSet for SSA

[gopherbot]

Change https://golang.org/cl/284899 mentions this issue: [dev.regabi] cmd/compile: replace ir.Name map with ir.NameSet for dwarf

[gopherbot]

Change https://golang.org/cl/285913 mentions this issue: [dev.regabi] cmd/compile: replace ir.Name map with ir.NameSet for SSA 2

[mdempsky]

This is fixed on dev.regabi. Thanks @oiooj!