Auto merge of #51833 - wesleywiser:faster_large_constant_arrays, r=oli-obk

Auto merge of #51833 - wesleywiser:faster_large_constant_arrays, r=oli-obk

Speed up compilation of large constant arrays

This is a different approach to #51672 as suggested by @oli-obk. Rather
than write each repeated value one-by-one, we write the first one and
then copy its value directly into the remaining memory.

With this change, the [toy program](https://github.com/rust-lang/rust/blob/c2f4744d2db4e162df824d0bd0b093ba4b351545/src/test/run-pass/mir_heavy_promoted.rs) goes from 63 seconds to 19 seconds on my machine.

Edit: Inlining `Size::bytes()` saves an additional 6 seconds dropping the total time to 13 seconds on my machine.

Edit2: Now down to 2.8 seconds.

r? @oli-obk

cc @nnethercote @eddyb