cmd/compile: regalloc: drop values that aren't used until after a call

cmd/compile: regalloc: drop values that aren't used until after a call

No point in keeping values in registers when their next use is after
a call, as we'd have to spill/restore them anyway.

cmd/go is 0.1% smaller.

Fixes #59297

Change-Id: I10ee761d0d23229f57de278f734c44d6a8dccd6c
Reviewed-on: https://go-review.googlesource.com/c/go/+/509255
LUCI-TryBot-Result: Go LUCI <golang-scoped@luci-project-accounts.iam.gserviceaccount.com>
Reviewed-by: Michael Pratt <mpratt@google.com>
Reviewed-by: Cherry Mui <cherryyz@google.com>