[dev.regabi] cmd/compile: use byte for CallExpr.Use
[dev.regabi] cmd/compile: use byte for CallExpr.Use

Reduce 16 byte for CallExpr, from 184 to 168 on 64-bit archs.

Passes toolstash -cmp.

Change-Id: I59c7609ccd03e8b4a7df8d2c30de8022ae312cee
Reviewed-on: https://go-review.googlesource.com/c/go/+/283732
Trust: Cuong Manh Le <54b4457f1dff6823379654eb5867596497ba782f@gmail.com>
Run-TryBot: Cuong Manh Le <54b4457f1dff6823379654eb5867596497ba782f@gmail.com>
Reviewed-by: Matthew Dempsky <e56ac2c500064424afac424d4b5e53bd0570a731@google.com>
TryBot-Result: Go Bot <66cb808b70d30c07676d5e946fee83fd561249e5@golang.org>

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