cmd/compile: don't require Heapaddr for heap vars in dead code

DWARF generation's createConservativeVar asserts that every variable
marked ir.EscHeap has a non-nil Heapaddr. This assumption only holds for
reachable declarations.

Escape analysis runs before SSA generation and is not reachability-aware,
so it can mark a variable declared in dead code as heap-escaped. During
SSA generation the ODCL for such a variable lands in an unreachable block,
where stmt() returns early and never calls newHeapaddr. The variable is
therefore left with Esc()==EscHeap but Heapaddr==nil, a legitimate state:
the allocation was never generated, so there is no heap address to point
at.

Only build the heap-deref location list when Heapaddr is present, and
otherwise emit a conservative variable with no location list, as was done
before CL 684377. A reachable heap-escaped variable always gets a Heapaddr
via newHeapaddr, so a nil Heapaddr uniquely identifies the dead-code case.

Fixes #80097.

Change-Id: I91e475d0256c0f5e4d2dfabcacff92904469087a
Reviewed-on: https://go-review.googlesource.com/c/go/+/792960
LUCI-TryBot-Result: golang-scoped@luci-project-accounts.iam.gserviceaccount.com <golang-scoped@luci-project-accounts.iam.gserviceaccount.com>
Reviewed-by: Cherry Mui <cherryyz@google.com>
Reviewed-by: Keith Randall <khr@google.com>
Reviewed-by: Keith Randall <khr@golang.org>