ssa: reuses slices for basicBlock.params

[mathetake]

This replaces the basicBlock.params field with the reusable
VarLength[Value] type. As a result, the compilation starts
using less memory and allocations.

goos: darwin
goarch: arm64
pkg: github.com/tetratelabs/wazero
                      │  old.txt   │             new.txt              │
                      │   sec/op   │   sec/op    vs base              │
Compilation/wazero-10   2.004 ± 2%   2.001 ± 0%       ~ (p=0.620 n=7)
Compilation/zig-10      4.164 ± 1%   4.174 ± 3%       ~ (p=0.097 n=7)
geomean                 2.888        2.890       +0.06%

                      │   old.txt    │              new.txt               │
                      │     B/op     │     B/op      vs base              │
Compilation/wazero-10   297.7Mi ± 0%   297.5Mi ± 0%  -0.06% (p=0.001 n=7)
Compilation/zig-10      594.0Mi ± 0%   593.9Mi ± 0%  -0.01% (p=0.001 n=7)
geomean                 420.5Mi        420.3Mi       -0.03%

                      │   old.txt   │              new.txt              │
                      │  allocs/op  │  allocs/op   vs base              │
Compilation/wazero-10   472.5k ± 0%   457.1k ± 0%  -3.25% (p=0.001 n=7)
Compilation/zig-10      277.2k ± 0%   275.7k ± 0%  -0.53% (p=0.001 n=7)
geomean                 361.9k        355.0k       -1.90%