cmd/compile: detect unsafe conversions from smaller to larger types

This CL extends the runtime instrumentation for (*T)(ptr) to also
check that the first and last bytes of *(*T)(ptr) are part of the same
heap object.

Updates #22218.
Updates #34959.

Change-Id: I2c8063fe1b7fe6e6145e41c5654cb64dd1c9dd41
Reviewed-on: https://go-review.googlesource.com/c/go/+/201778


Run-TryBot: Matthew Dempsky <mdempsky@google.com>
Reviewed-by: Keith Randall <khr@golang.org>
Reviewed-by: Cuong Manh Le <cuong.manhle.vn@gmail.com>
TryBot-Result: Gobot Gobot <gobot@golang.org>