cmd/compile: fix rewriting slice literal call argument

When seeing Key:Value expression in slice literal, the compiler only
needs to emit tmp var for the Value, not the whole expression.

Fixes #49240

Change-Id: I7bda3c796a93c0fa1974f7c5930f38025dfa665c
Reviewed-on: https://go-review.googlesource.com/c/go/+/360055


Trust: Cuong Manh Le <cuong.manhle.vn@gmail.com>
Run-TryBot: Cuong Manh Le <cuong.manhle.vn@gmail.com>
TryBot-Result: Go Bot <gobot@golang.org>
Reviewed-by: Cherry Mui <cherryyz@google.com>
Reviewed-by: Matthew Dempsky <mdempsky@google.com>