Eliminate dead variables from dehydrated data.

When a local variable is eliminated during optimization, we set its
storage type to `kEliminated`. The dehydrator omits these variables and
their names from its output.

This is easy to do, shrinks our binary size, and saves some memory.

Change-Id: I425adc67450f8d744b51a711b5124f00e7dfd5c4
Reviewed-on: https://skia-review.googlesource.com/c/skia/+/562309
Auto-Submit: John Stiles <johnstiles@google.com>
Reviewed-by: Brian Osman <brianosman@google.com>
Commit-Queue: Brian Osman <brianosman@google.com>
4 files changed