cmd/compile: fold handling OCONV logic to separate function
cmd/compile: fold handling OCONV logic to separate function

So next CL can re-use the logic to perform checkptr instrumentation.

Change-Id: I3241e9c3c84da04db71fd1d4fd83cb76b2e18521
Reviewed-on: https://go-review.googlesource.com/c/go/+/345435
Trust: Cuong Manh Le <54b4457f1dff6823379654eb5867596497ba782f@gmail.com>
Run-TryBot: Cuong Manh Le <54b4457f1dff6823379654eb5867596497ba782f@gmail.com>
TryBot-Result: Go Bot <66cb808b70d30c07676d5e946fee83fd561249e5@golang.org>
Reviewed-by: Matthew Dempsky <e56ac2c500064424afac424d4b5e53bd0570a731@google.com>

CAFxX/go,CAFxX/go,cloudflare/go,nawawi/go,golang/go,CAFxX/go,nawawi/go,cloudflare/go,CAFxX/go,nawawi/go,nawawi/go,golang/go,nawawi/go,golang/go,CAFxX/go,golang/go,nawawi/go,CAFxX/go,golang/go,CAFxX/go,cloudflare/go,golang/go,cloudflare/go,golang/go,cloudflare/go,nawawi/go,nawawi/go,cloudflare/go,golang/go,cloudflare/go,cloudflare/go,CAFxX/go