cmd/compile/internal/amd64: simplify assembly generator
cmd/compile/internal/amd64: simplify assembly generator

Merge two case-statements together, since they have similar logic.

1. That makes the assembly generator more clear.
2. The total size of cmd/compile decreases about 0.8KB.

Change-Id: I0144a07152202ee7b21e323bcd5dea80a351a6e3
Reviewed-on: https://go-review.googlesource.com/134215
Run-TryBot: Ben Shi <d305faf1f9df9cf68436458fcb54165e72b1642e@163.com>
TryBot-Result: Gobot Gobot <66cb808b70d30c07676d5e946fee83fd561249e5@golang.org>
Reviewed-by: Keith Randall <8c99c3a9284e493be632950b84cd789d08ed3e9d@golang.org>

nawawi/go,cloudflare/go,cloudflare/go,golang/go,josharian/go,golang/go,nawawi/go,CAFxX/go,CAFxX/go,akutz/go,nawawi/go,akutz/go,josharian/go,cloudflare/go,cloudflare/go,golang/go,golang/go,akutz/go,nawawi/go,CAFxX/go,cloudflare/go,akutz/go,josharian/go,CAFxX/go,cloudflare/go,CAFxX/go,golang/go,CAFxX/go,golang/go,cloudflare/go,cloudflare/go,josharian/go,josharian/go,josharian/go,nawawi/go,akutz/go,CAFxX/go,golang/go,nawawi/go,josharian/go,CAFxX/go,akutz/go,nawawi/go,josharian/go,akutz/go,akutz/go,nawawi/go,golang/go