cmd/compile/internal/gc: fix invalid positions for sink nodes in esc.go

Make OAS2 and OAS2FUNC sink locations point to the assignment position,
not the nth LHS position.

Fixes #26987

Change-Id: Ibeb9df2da754da8b6638fe1e49e813f37515c13c
Reviewed-on: https://go-review.googlesource.com/129315


Run-TryBot: Iskander Sharipov <iskander.sharipov@intel.com>
TryBot-Result: Gobot Gobot <gobot@golang.org>
Reviewed-by: David Chase <drchase@google.com>
Reviewed-by: Cherry Zhang <cherryyz@google.com>