cmd/compile: handle degenerate entry blocks in -N debug gen

The code that created DWARF debug var locations for input parameters
in the non-optimized case for regabi was not doing the right thing for
degenerate functions with infinite loops. Detect these cases and don't
try to emit the normal location data.

Fixes #45948.

Change-Id: I2717fc4bac2e03d5d850a6ec8a09ed05fed0c896
Reviewed-on: https://go-review.googlesource.com/c/go/+/316752


Trust: Than McIntosh <thanm@google.com>
Run-TryBot: Than McIntosh <thanm@google.com>
Reviewed-by: Cherry Zhang <cherryyz@google.com>
TryBot-Result: Go Bot <gobot@golang.org>