dep_graph values are unstable

[klensy]

See the recent run https://perf.rust-lang.org/compare.html?start=a8e1186e3c14a54f7a38cc1183117dc7e77f4f82&end=9fa9ef385c0aad8f5d4c8f7d92dca474367943a3&stat=size%3Adep_graph&tab=compile&nonRelevant=true&showRawData=true and expand historical data: few bytes up and down all the time.

rust-lang/rust#131634 (comment).

[Kobzol]

Not sure if this is an actual issue or just some harmless artifact, but if anything, this should probably be filed as an issue against the compiler, not rustc-perf. (unless the compiler's number is stable and we somehow record it wrongly into the DB, but that seems unlikely)