[WebAssembly] Enable loop unrolling

Enable partial and runtime unrolling with a threshold of 30, which
was derived from a large number of kernels running on node and
wasmtime for amd64 and aarch64.

Unrolling is enabled by default at -O2 and -O3 and is disabled at
-Oz and -Os. Compiling with -Os is recommended if the wasm binary
size is the most important factor.

Differential Revision: https://reviews.llvm.org/D95125