[dev.regabi] cmd/compile: change AddrExpr.Alloc to AddrExpr.Prealloc

[dev.regabi] cmd/compile: change AddrExpr.Alloc to AddrExpr.Prealloc

For being consistent with other Prealloc fields.

[git-generate]

cd src/cmd/compile/internal/ir
rf '
  mv AddrExpr.Alloc AddrExpr.Prealloc
'
go generate

Change-Id: Id1b05119092036e3f8208b73b63bd0ca6ceb7b15
Reviewed-on: https://go-review.googlesource.com/c/go/+/279450
Trust: Cuong Manh Le <cuong.manhle.vn@gmail.com>
Reviewed-by: Matthew Dempsky <mdempsky@google.com>
Run-TryBot: Cuong Manh Le <cuong.manhle.vn@gmail.com>
TryBot-Result: Go Bot <gobot@golang.org>