cmd/compile/internal/gc: cleanup walkselectcases slightly
cmd/compile/internal/gc: cleanup walkselectcases slightly

Remove some unnecessary code. Most significantly, we can skip testing
"if ch == nil { block() }", because this is already the semantics
implied by normal send/receive operations.

Updates #40410.

Change-Id: I4acd33383cc876719fc3b998d85244d4ac1ff9d9
Reviewed-on: https://go-review.googlesource.com/c/go/+/245126
Run-TryBot: Matthew Dempsky <e56ac2c500064424afac424d4b5e53bd0570a731@google.com>
TryBot-Result: Gobot Gobot <66cb808b70d30c07676d5e946fee83fd561249e5@golang.org>
Reviewed-by: Cuong Manh Le <54b4457f1dff6823379654eb5867596497ba782f@gmail.com>

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