[dev.regabi] cmd/compile: stop analyze NameOffsetExpr.Name_ in escape analysis

[dev.regabi] cmd/compile: stop analyze NameOffsetExpr.Name_ in escape analysis

It is always used with global variables, so we can skip analyze it, the
same as what we are doing for ONAME/PEXTERN nodes.

While at it, add a Fatalf check to ensure NewNameOffsetExpr is only
called for global variables.

For #43737

Change-Id: Iac444ed8d583baba5042bea096531301843b1e8f
Reviewed-on: https://go-review.googlesource.com/c/go/+/284118
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>