regalloc: removes map from RegAllocFunction

[mathetake]

This improves the compilation slightly as below:

wazero compiled as a wasip1

goos: darwin
goarch: arm64
pkg: github.com/tetratelabs/wazero
               │ old_zig.txt │           new_zig.txt            │
               │   sec/op    │   sec/op    vs base              │
Compilation-10    2.227 ± 0%   2.184 ± 1%  -1.96% (p=0.001 n=7)

               │ old_zig.txt  │            new_zig.txt             │
               │     B/op     │     B/op      vs base              │
Compilation-10   337.2Mi ± 0%   337.3Mi ± 0%  +0.02% (p=0.001 n=7)

               │ old_zig.txt │            new_zig.txt            │
               │  allocs/op  │  allocs/op   vs base              │
Compilation-10   593.5k ± 0%   592.9k ± 0%  -0.10% (p=0.001 n=7)

Zig stdlib

goos: darwin
goarch: arm64
pkg: github.com/tetratelabs/wazero
               │  old.txt   │             new.txt              │
               │   sec/op   │   sec/op    vs base              │
Compilation-10   4.371 ± 1%   4.329 ± 0%  -0.96% (p=0.001 n=7)

               │   old.txt    │              new.txt               │
               │     B/op     │     B/op      vs base              │
Compilation-10   599.3Mi ± 0%   599.3Mi ± 0%  -0.00% (p=0.002 n=7)

               │   old.txt   │           new.txt            │
               │  allocs/op  │  allocs/op   vs base         │
Compilation-10   287.9k ± 0%   287.9k ± 0%  ~ (p=0.053 n=7)

#2182