validation: avoids allocation per br_table

[mathetake]

This eliminates the allocation that previously happened
per br_table instruction during function validation.

goos: darwin
goarch: arm64
pkg: github.com/tetratelabs/wazero
                      │  old.txt   │             new.txt              │
                      │   sec/op   │   sec/op    vs base              │
Compilation/wazero-10   1.993 ± 0%   1.995 ± 0%       ~ (p=0.485 n=6)
Compilation/zig-10      4.161 ± 1%   4.169 ± 0%       ~ (p=0.699 n=6)
Compilation/zz-10       18.57 ± 0%   18.55 ± 0%       ~ (p=0.240 n=6)
geomean                 5.360        5.363       +0.06%

                      │   old.txt    │              new.txt               │
                      │     B/op     │     B/op      vs base              │
Compilation/wazero-10   287.1Mi ± 0%   286.8Mi ± 0%  -0.13% (p=0.002 n=6)
Compilation/zig-10      590.3Mi ± 0%   590.3Mi ± 0%  -0.01% (p=0.002 n=6)
Compilation/zz-10       553.7Mi ± 0%   549.4Mi ± 0%  -0.78% (p=0.002 n=6)
geomean                 454.4Mi        453.0Mi       -0.31%

                      │   old.txt   │              new.txt              │
                      │  allocs/op  │  allocs/op   vs base              │
Compilation/wazero-10   449.1k ± 0%   445.3k ± 0%  -0.84% (p=0.002 n=6)
Compilation/zig-10      273.8k ± 0%   273.0k ± 0%  -0.30% (p=0.002 n=6)
Compilation/zz-10       830.9k ± 0%   783.3k ± 0%  -5.72% (p=0.002 n=6)
geomean                 467.5k        456.7k       -2.32%

#2182