compiler: memory usage optimization around br_table

[mathetake]

This optimizes the memory usage during compilation for
br_table instructions. As you can see in the bench results below,
for some cases where lots of br_tables exists (the case named zz),
the compilation uses 10% less allocations and 5% less memory, hence
the slightly faster compilation.

goos: darwin
goarch: arm64
pkg: github.com/tetratelabs/wazero
                      │  old.txt   │             new.txt              │
                      │   sec/op   │   sec/op    vs base              │
Compilation/wazero-10   2.015 ± 2%   1.993 ± 0%  -1.09% (p=0.002 n=6)
Compilation/zig-10      4.200 ± 0%   4.161 ± 1%  -0.93% (p=0.004 n=6)
Compilation/zz-10       18.70 ± 0%   18.57 ± 0%  -0.69% (p=0.002 n=6)
geomean                 5.409        5.360       -0.90%

                      │   old.txt    │              new.txt               │
                      │     B/op     │     B/op      vs base              │
Compilation/wazero-10   297.5Mi ± 0%   287.1Mi ± 0%  -3.48% (p=0.002 n=6)
Compilation/zig-10      593.9Mi ± 0%   590.3Mi ± 0%  -0.61% (p=0.002 n=6)
Compilation/zz-10       582.6Mi ± 0%   553.7Mi ± 0%  -4.96% (p=0.002 n=6)
geomean                 468.7Mi        454.4Mi       -3.03%

                      │   old.txt   │              new.txt               │
                      │  allocs/op  │  allocs/op   vs base               │
Compilation/wazero-10   457.0k ± 0%   449.1k ± 0%   -1.72% (p=0.002 n=6)
Compilation/zig-10      275.8k ± 0%   273.8k ± 0%   -0.70% (p=0.002 n=6)
Compilation/zz-10       926.5k ± 0%   830.9k ± 0%  -10.32% (p=0.002 n=6)
geomean                 488.7k        467.5k        -4.35%

#2182