cmd/compile: preserve name association when eliding copies in expand_calls
cmd/compile: preserve name association when eliding copies in expand_calls

If v is a Copy of x, we will rewrite v to x. If v has a name
associated to it, let the name associate to x.

Under register ABI, this helps associate in-register Arg values
to the parameters' names. (But does not address all cases.)

Change-Id: I47c779e56c9d0823a88890497e32326bc0290f82
Reviewed-on: https://go-review.googlesource.com/c/go/+/309330
Trust: Cherry Zhang <d62e63aa42ce272d7b6a5055d97e942b33a34679@google.com>
Run-TryBot: Cherry Zhang <d62e63aa42ce272d7b6a5055d97e942b33a34679@google.com>
TryBot-Result: Go Bot <66cb808b70d30c07676d5e946fee83fd561249e5@golang.org>
Reviewed-by: David Chase <ae171625adc8645b8b79b7e405a6da5aba5341e0@google.com>
Reviewed-by: Than McIntosh <4b2922593166fc595a9b1a408f34f5d3817fe9d2@google.com>

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