Fixes compiler performance in mega_example.

[esdrubal]

Description

ReplaceDecls for method application was calling replace_decls twice for FunctionDecl. This was visiting function declarations as a binary tree, doing replace decls twice per FunctionDecl.

With these changes, we merge both decl mappings before calling replace_decls once instead of twice.

The performance dropped from over 2min(did not wait for test to finish) to 7 seconds in the mega example. With --experimental-new-encoding and --release.

Checklist

• I have linked to any relevant issues.
• I have commented my code, particularly in hard-to-understand areas.
• I have updated the documentation where relevant (API docs, the reference, and the Sway book).
  • If my change requires substantial documentation changes, I have requested support from the DevRel team
• I have added tests that prove my fix is effective or that my feature works.
• I have added (or requested a maintainer to add) the necessary Breaking* or New Feature labels where relevant.
• I have done my best to ensure that my PR adheres to the Fuel Labs Code Review Standards.
• I have requested a review from the relevant team or maintainers.

[github-actions]

Benchmark for 0c46975

Click to view benchmark

Test	Base	PR	%
code_action	5.4±0.39ms	5.6±0.17ms	+3.70%
code_lens	306.8±5.67ns	288.6±10.28ns	-5.93%
compile	6.3±0.07s	6.3±0.09s	0.00%
completion	4.9±0.10ms	5.0±0.13ms	+2.04%
did_change_with_caching	6.1±0.13s	5.9±0.07s	-3.28%
document_symbol	991.0±48.77µs	927.5±12.22µs	-6.41%
format	77.8±1.24ms	77.1±2.03ms	-0.90%
goto_definition	363.5±6.58µs	368.2±13.69µs	+1.29%
highlight	8.8±0.10ms	9.1±0.15ms	+3.41%
hover	603.1±13.13µs	609.8±28.01µs	+1.11%
idents_at_position	123.2±0.34µs	122.4±2.39µs	-0.65%
inlay_hints	648.3±23.87µs	660.3±13.02µs	+1.85%
on_enter	485.0±12.69ns	503.0±27.87ns	+3.71%
parent_decl_at_position	3.6±0.05ms	3.7±0.03ms	+2.78%
prepare_rename	365.1±7.45µs	369.9±10.42µs	+1.31%
rename	9.3±0.15ms	9.9±0.24ms	+6.45%
semantic_tokens	1012.3±59.62µs	1047.5±12.64µs	+3.48%
token_at_position	360.2±2.95µs	366.4±2.64µs	+1.72%
tokens_at_position	3.6±0.06ms	3.8±0.03ms	+5.56%
tokens_for_file	425.5±4.81µs	415.9±10.59µs	-2.26%
traverse	52.4±2.29ms	51.1±1.54ms	-2.48%

[github-actions]

Benchmark for 0575c7b

Click to view benchmark

Test	Base	PR	%
code_action	5.3±0.11ms	5.6±0.24ms	+5.66%
code_lens	294.7±13.19ns	289.5±11.66ns	-1.76%
compile	6.2±0.08s	6.2±0.08s	0.00%
completion	4.9±0.22ms	5.5±0.48ms	+12.24%
did_change_with_caching	6.2±0.06s	6.3±0.06s	+1.61%
document_symbol	937.4±18.65µs	1044.3±41.20µs	+11.40%
format	79.0±0.81ms	77.2±1.58ms	-2.28%
goto_definition	369.1±6.68µs	369.4±13.19µs	+0.08%
highlight	8.8±0.32ms	9.4±0.32ms	+6.82%
hover	606.4±7.37µs	607.2±15.76µs	+0.13%
idents_at_position	122.5±0.69µs	122.7±1.78µs	+0.16%
inlay_hints	657.4±30.27µs	679.7±35.68µs	+3.39%
on_enter	496.8±21.22ns	513.1±14.47ns	+3.28%
parent_decl_at_position	3.6±0.07ms	3.8±0.12ms	+5.56%
prepare_rename	364.9±6.82µs	368.8±16.59µs	+1.07%
rename	9.4±0.27ms	10.0±0.52ms	+6.38%
semantic_tokens	1024.0±23.69µs	1057.6±12.99µs	+3.28%
token_at_position	360.0±5.83µs	363.8±5.52µs	+1.06%
tokens_at_position	3.6±0.09ms	3.8±0.09ms	+5.56%
tokens_for_file	412.9±6.09µs	414.4±4.40µs	+0.36%
traverse	51.0±2.27ms	51.4±1.55ms	+0.78%