[dev.regabi] cmd/compile: change ir.IsAssignable -> ir.IsAddressable

ir.IsAssignable does not include map index expression, so it should be
named ir.IsAddressable instead.

[git-generate]

cd src/cmd/compile/internal/ir
rf '
  mv IsAssignable IsAddressable
'

Change-Id: Ief6188e7b784ba9592d7b0cbec33b5f70d78f638
Reviewed-on: https://go-review.googlesource.com/c/go/+/279436


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: Matthew Dempsky <mdempsky@google.com>