The ssa backend is aggressive about placing constants and
certain other values in the Entry block.  It's implausible
that the original line numbers for these constants makes
any sort of sense when it appears to a user stepping in a
debugger, and they're also not that useful in dumps since
entry-block instructions tend to be constants (i.e.,
unlikely to be the cause of a crash).

Therefore, use src.NoXPos for any values that are explicitly
inserted into a function's entry block.

Passes all tests, including ssa/debug_test.go with both
gdb and a fairly recent dlv.  Hand-verified that it solves
the reported problem; constructed a test that reproduced
a problem, and fixed it.

Modified test harness to allow injection of slightly more
interesting inputs.

Fixes #22558.

Change-Id: I4476927067846bc4366da7793d2375c111694c55
Reviewed-on: https://go-review.googlesource.com/81215
Run-TryBot: David Chase <drchase@google.com>
TryBot-Result: Gobot Gobot <gobot@golang.org>
Reviewed-by: Keith Randall <khr@golang.org>