cmd/compile: assume unsafe pointer arithmetic generates non-nil results

cmd/compile: assume unsafe pointer arithmetic generates non-nil results

I've never seen a case where unsafe arithmetic is used to generate a nil.
(Something like var x uintptr; unsafe.Pointer(x - x).)
We can assume that if someone is doing arithmetic with pointers, the
result will be non-nil. Our unsafe rules already forbid this, although
we should be more explicit.

RELNOTE=It is invalid to convert a nil unsafe.Pointer to uintptr and back, with arithmetic.
(This was already invalid, but this statement has been added for clarification.)

Fixes #27180

Change-Id: I1880b7725a9fd99e4613799930fdad9aaa99e8f0
Reviewed-on: https://go-review.googlesource.com/c/146058
Reviewed-by: Austin Clements <austin@google.com>