wazevo(arm64): optimize out unnecessary UExtend

[mathetake]

$ benchstat old.txt new.txt
goos: darwin
goarch: arm64
pkg: github.com/tetratelabs/wazero/internal/integration_test/stdlibs
                                        │  old.txt   │              new.txt              │
                                        │   sec/op   │   sec/op    vs base               │
Zig/optimizing/Run/test-opt.wasm-10       20.62 ± 2%   20.18 ± 1%  -2.14% (p=0.001 n=10)
Zig/optimizing/Compile/test-opt.wasm-10   6.292 ± 2%   6.108 ± 3%  -2.92% (p=0.007 n=10)
Zig/optimizing/Run/test.wasm-10           22.39 ± 3%   21.15 ± 1%  -5.53% (p=0.000 n=10)
Zig/optimizing/Compile/test.wasm-10       6.035 ± 2%   6.171 ± 2%  +2.26% (p=0.000 n=10)
geomean                                   11.51        11.26       -2.12%

[mathetake]

fuzzing now