cmd/compile: change unsafeUintptrTag from var to const
cmd/compile: change unsafeUintptrTag from var to const

Change-Id: Ie30878199e24cce5b75428e6b602c017ebd16642
Reviewed-on: https://go-review.googlesource.com/102175
Run-TryBot: Matthew Dempsky <e56ac2c500064424afac424d4b5e53bd0570a731@google.com>
Reviewed-by: Daniel Martí <29bf28b42a938ace12a5819da4ed4ba03d82f315@mvdan.cc>

golang/go,akutz/go,akutz/go,CAFxX/go,golang/go,josharian/go,CAFxX/go,cloudflare/go,akutz/go,CAFxX/go,Cofyc/go,CAFxX/go,nawawi/go,nawawi/go,josharian/go,CAFxX/go,golang/go,josharian/go,CAFxX/go,cloudflare/go,golang/go,cloudflare/go,akutz/go,akutz/go,cloudflare/go,Cofyc/go,CAFxX/go,golang/go,golang/go,Cofyc/go,josharian/go,akutz/go,josharian/go,Cofyc/go,golang/go,Cofyc/go,Cofyc/go,golang/go,cloudflare/go,nawawi/go,nawawi/go,josharian/go,akutz/go,CAFxX/go,Cofyc/go,cloudflare/go,akutz/go,nawawi/go,Cofyc/go,nawawi/go,josharian/go,cloudflare/go,nawawi/go,cloudflare/go,nawawi/go,josharian/go