[llvm] [RISCV] Enable -riscv-enable-sink-fold by default. (PR #82026)

[Wang Pengcheng via llvm-commits]

wangpc-pp wrote:

> Looks like this patch causes ~22.8% performance regression on `MultiSource/Benchmarks/MallocBench/gs/gs`, which is compiled with `-Os`.
> 
> See also [dtcxzyw/llvm-ci#1073](https://github.com/dtcxzyw/llvm-ci/issues/1073) and [dtcxzyw/llvm-ci#1098](https://github.com/dtcxzyw/llvm-ci/pull/1098).

So maybe we should disable this pass when optimizing for size?

https://github.com/llvm/llvm-project/pull/82026