cmd/compile/internal/escape: mark blankLoc as transient

cmd/compile/internal/escape: mark blankLoc as transient

Discarded values never persist, so they can be transiently allocated
too.

Change-Id: I036ce0c1eea45e437142497bb7df3ecb44b56e52
Reviewed-on: https://go-review.googlesource.com/c/go/+/520256
Auto-Submit: Matthew Dempsky <mdempsky@google.com>
Reviewed-by: Than McIntosh <thanm@google.com>
TryBot-Result: Gopher Robot <gobot@golang.org>
Reviewed-by: Cuong Manh Le <cuong.manhle.vn@gmail.com>
Run-TryBot: Matthew Dempsky <mdempsky@google.com>