Perform a program clone at the end of parsing to remove any unreachable AST nodes.
Actually fixing the parser to never create these looks like a huge amount of work.
Fixed: tint:749
Change-Id: Ib956634257e0933c9702d6be22f79942f7cf4c51
Reviewed-on: https://dawn-review.googlesource.com/c/tint/+/49520
Kokoro: Kokoro <noreply+kokoro@google.com>
Reviewed-by: James Price <jrprice@google.com>
Commit-Queue: Ben Clayton <bclayton@google.com>
Ben Clayton