ssa: optimizes slice allocations

[mathetake]

This makes the compilation faster and use less memory:

goos: darwin
goarch: arm64
pkg: github.com/tetratelabs/wazero
                      │  old.txt   │             new.txt              │
                      │   sec/op   │   sec/op    vs base              │
Compilation/wazero-10   2.184 ± 0%   2.110 ± 0%  -3.40% (p=0.001 n=7)
Compilation/zig-10      4.331 ± 1%   4.187 ± 1%  -3.31% (p=0.001 n=7)
geomean                 3.075        2.972       -3.36%

                      │   old.txt    │               new.txt               │
                      │     B/op     │     B/op      vs base               │
Compilation/wazero-10   337.3Mi ± 0%   301.9Mi ± 0%  -10.49% (p=0.001 n=7)
Compilation/zig-10      599.3Mi ± 0%   594.3Mi ± 0%   -0.84% (p=0.001 n=7)
geomean                 449.6Mi        423.6Mi        -5.79%

                      │   old.txt   │              new.txt               │
                      │  allocs/op  │  allocs/op   vs base               │
Compilation/wazero-10   592.9k ± 0%   527.9k ± 0%  -10.97% (p=0.001 n=7)
Compilation/zig-10      287.8k ± 0%   278.6k ± 0%   -3.20% (p=0.001 n=7)
geomean                 413.1k        383.5k        -7.17%

#2182