cmd/compile: refactor walkIndexMap

cmd/compile: refactor walkIndexMap

So all runtime map functions will use the same code path, make it easier
for future CL to handle stack object for map key.

Passes toolstash -cmp.

For #43753

Change-Id: I374fa4e351c1eba079e2ccb637b1ef5adad1488f
Reviewed-on: https://go-review.googlesource.com/c/go/+/321713
Trust: Cuong Manh Le <cuong.manhle.vn@gmail.com>
Reviewed-by: Keith Randall <khr@golang.org>
Reviewed-by: Cherry Mui <cherryyz@google.com>
Run-TryBot: Cuong Manh Le <cuong.manhle.vn@gmail.com>
TryBot-Result: Gopher Robot <gobot@golang.org>