Auto merge of #136593 - lukas-code:ty-value-perf, r=oli-obk

valtree performance tuning

Summary: This PR makes type checking of code with many type-level constants faster.

After rust-lang/rust#136180 was merged, we observed a small perf regression (rust-lang/rust#136318 (comment)). This happened because that PR introduced additional copies in the fast reject code path for consts, which is very hot for certain crates: https://github.com/rust-lang/rust/blob/6c1d960d88dd3755548b3818630acb63fa98187e/compiler/rustc_type_ir/src/fast_reject.rs#L486-L487

This PR improves the performance again by properly interning the valtrees so that copying and comparing them becomes faster. This will become especially useful with `feature(adt_const_params)`, so the fast reject code doesn't have to do a deep compare of the valtrees.

Note that we can't just compare the interned consts themselves in the fast reject, because sometimes `'static` lifetimes in the type are be replaced with inference variables (due to canonicalization) on one side but not the other.

A less invasive alternative that I considered is simply avoiding copies introduced by rust-lang/rust#136180 and comparing the valtrees it in-place (see commit: rust-lang/rust@9e91e50 / perf results: rust-lang/rust#136593 (comment)), however that was still measurably slower than interning.

There are some minor regressions in secondary benchmarks: These happen due to changes in memory allocations and seem acceptable to me. The crates that make heavy use of valtrees show no significant changes in memory usage.

Author: bors