cmd/compile: On a runtime.KeepAlive call, keep whole variable alive

We generate an OpKeepAlive for the idata portion of the interface
for a runtime.KeepAlive call.  But given such an op, we need to keep
the entire containing variable alive, not just the range that was
passed to the OpKeepAlive operation.

Fixes #17710

Change-Id: I90de66ec8065e22fb09bcf9722999ddda289ae6e
Reviewed-on: https://go-review.googlesource.com/32477


Run-TryBot: Keith Randall <khr@golang.org>
Reviewed-by: Joe Tsai <thebrokentoaster@gmail.com>