ssa: reuses slice on basicBlock.loopNestingForestChildren

[mathetake]

loopNestingForestChildren is non-empty only when the basic block
is a loop header, and hence previously the slice hasn't been reused much.

This resolves it, and as a result, the compilation uses slightly less memory.

goos: darwin
goarch: arm64
pkg: github.com/tetratelabs/wazero
               │  old.txt   │           new.txt           │
               │   sec/op   │   sec/op    vs base         │
Compilation-10   2.423 ± 0%   2.436 ± 2%  ~ (p=0.097 n=7)

               │   old.txt    │              new.txt               │
               │     B/op     │     B/op      vs base              │
Compilation-10   340.3Mi ± 0%   339.9Mi ± 0%  -0.10% (p=0.001 n=7)

               │   old.txt   │              new.txt              │
               │  allocs/op  │  allocs/op   vs base              │
Compilation-10   605.5k ± 0%   603.9k ± 0%  -0.27% (p=0.001 n=7)

#2182