runtime/pprof/internal/profile: use idiomatic swapping
runtime/pprof/internal/profile: use idiomatic swapping

gogrep found only one such case with the pattern below:

	$tmp := $x; $x = $y; $y = $tmp

R=1.13

Change-Id: I6e46fb5ef2887f24fa9fc451323a8cef272e2886
Reviewed-on: https://go-review.googlesource.com/c/151200
Run-TryBot: Bryan C. Mills <1c8aad60184261ccede67f5c63a0d2a3bf3c9ff4@google.com>
TryBot-Result: Gobot Gobot <66cb808b70d30c07676d5e946fee83fd561249e5@golang.org>
Reviewed-by: Bryan C. Mills <1c8aad60184261ccede67f5c63a0d2a3bf3c9ff4@google.com>

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