cmd/compile: don't attach lines to SB, SP, similar constants
cmd/compile: don't attach lines to SB, SP, similar constants

Attaching positions to SB, SP, initial mem can result in
less-good line-numbering when compiled for debugging.
This "fix" also removes source position from a zero-valued
struct (but not from its fields) and from a zero-length
array constant.

This may be a general problem for constants in entry blocks.

Fixes #20367.

Change-Id: I7e9df3341be2e2f60f127d35bb31e43cdcfce9a1
Reviewed-on: https://go-review.googlesource.com/43531
Run-TryBot: David Chase <ae171625adc8645b8b79b7e405a6da5aba5341e0@google.com>
TryBot-Result: Gobot Gobot <66cb808b70d30c07676d5e946fee83fd561249e5@golang.org>
Reviewed-by: Keith Randall <8c99c3a9284e493be632950b84cd789d08ed3e9d@golang.org>

Cofyc/go,polyverse-security/go,vsekhar/elastic-go,CAFxX/go,josharian/go,vsekhar/elastic-go,Cofyc/go,vsekhar/elastic-go,josharian/go,codestation/go,derekmarcotte/go,Cofyc/go,nawawi/go,ImJasonH/go,codestation/go,josharian/go,nawawi/go,Cofyc/go,ImJasonH/go,akutz/go,CAFxX/go,josharian/go,Cofyc/go,cloudflare/go,ImJasonH/go,josharian/go,golang/go,cloudflare/go,golang/go,codestation/go,CAFxX/go,polyverse-security/go,nawawi/go,CAFxX/go,codestation/go,nawawi/go,vsekhar/elastic-go,polyverse-security/go,derekmarcotte/go,ImJasonH/go,golang/go,golang/go,josharian/go,derekmarcotte/go,Cofyc/go,polyverse-security/go,josharian/go,polyverse-security/go,codestation/go,cloudflare/go,polyverse-security/go,akutz/go,cloudflare/go,nawawi/go,cloudflare/go,codestation/go,vsekhar/elastic-go,CAFxX/go,Cofyc/go,golang/go,vsekhar/elastic-go,josharian/go,cloudflare/go,nawawi/go,ImJasonH/go,ImJasonH/go,golang/go,christopher-henderson/Go,nawawi/go,cloudflare/go,derekmarcotte/go,ImJasonH/go,akutz/go,akutz/go,derekmarcotte/go,derekmarcotte/go,ImJasonH/go,derekmarcotte/go,golang/go,CAFxX/go,CAFxX/go,nawawi/go,codestation/go,akutz/go,polyverse-security/go,akutz/go,cloudflare/go,akutz/go,codestation/go,derekmarcotte/go,polyverse-security/go,CAFxX/go,vsekhar/elastic-go,golang/go,akutz/go,vsekhar/elastic-go,Cofyc/go