[dev.regabi] cmd/compile: delete n.List after collapsing OADDSTR to OLITERAL

[dev.regabi] cmd/compile: delete n.List after collapsing OADDSTR to OLITERAL

The leftover n.List is clearly unnecessary, but it makes the
inlining cost of the expression unnecessarily high.

This change breaks toolstash -cmp:

	# cmd/internal/src
	toolstash: compiler output differs, with optimizers disabled (-N)

	inconsistent log line:
	/tmp/go-build866291351/b230/_pkg_.a.log:77:
		/Users/rsc/go/src/cmd/internal/src/pos.go:275:6: can inline (*PosBase).SymFilename with cost 9 as: method(*PosBase) func() string { if b != nil { return b.symFilename }; return "gofile..??" }
	/tmp/go-build866291351/b230/_pkg_.a.stash.log:77:
		/Users/rsc/go/src/cmd/internal/src/pos.go:275:6: can inline (*PosBase).SymFilename with cost 11 as: method(*PosBase) func() string { if b != nil { return b.symFilename }; return "gofile..??" }

Separated from other constant work so that the bigger CL can pass toolstash -cmp.

Change-Id: I5c7ddbc8373207b5b9824eafb8639488da0ca1b7
Reviewed-on: https://go-review.googlesource.com/c/go/+/272868
Trust: Russ Cox <rsc@golang.org>
Run-TryBot: Russ Cox <rsc@golang.org>
TryBot-Result: Go Bot <gobot@golang.org>
Reviewed-by: Matthew Dempsky <mdempsky@google.com>