feat: flatten nested if-else statements with equivalent conditions

[TomAFrench]

Description

Problem*

Resolves #6786

Summary*

The new way of handling predicates on store instructions was resulting in us having a stack of load->if-else ->store->load ->if-else->store where the else value of each if-else was the previous if-else. This means we get a huge stack of nested if-else instructions which share the same then_condition so they can be squashed down to just have the top "then" and bottom "else".

I've solved this by having the simplify instruction inspect the then_value and else_value of any if-else instructions to see if we're building a nested structure and then flatten it automatically.

This PR drops the size of stateful-test-contract (after being reduced to just having the create_note function) from 31.2MB to 12.2MB.

Additional Context

Documentation*

Check one:

• No documentation needed.
• Documentation included in this PR.
• [For Experimental Features] Documentation to be submitted in a separate PR.

PR Checklist*

• I have tested the changes locally.
• I have formatted the changes with Prettier and/or cargo fmt on default settings.

[TomAFrench]

Need to modify some test names. Actually, they're mislabeled still.

[github-actions]

Changes to circuit sizes

Generated at commit: dc2c55c515631d71665fe3edbe44df2a8d5e0a70, compared to commit: ee0754b1c6b36961c180901db59dd593c183de77

🧾 Summary (10% most significant diffs)

Program	ACIR opcodes (+/-)	%	Circuit size (+/-)	%
fold_call_witness_condition	+1 ❌	+20.00%	+1 ❌	+5.26%
conditional_1	+108 ❌	+1.21%	+132 ❌	+0.72%

---

Full diff report 👇

Program	ACIR opcodes (+/-)	%	Circuit size (+/-)	%
fold_call_witness_condition	6 (+1)	+20.00%	20 (+1)	+5.26%
conditional_1	9,063 (+108)	+1.21%	18,397 (+132)	+0.72%
regression_5252	32,916 (0)	0.00%	44,519 (-3)	-0.01%

[github-actions]

Peak Memory Sample

Program	Peak Memory
keccak256	78.56M
workspace	123.40M
regression_4709	423.67M
ram_blowup_regression	1.58G
private-kernel-tail	207.23M
private-kernel-reset	720.82M
private-kernel-inner	292.44M
parity-root	172.24M

[github-actions]

Execution Sample

Program	Execution Time	%
sha256_regression	0m0.640s	1%
regression_4709	0m0.429s	8%
ram_blowup_regression	0m4.411s	1%
rollup-base-public	0m21.025s	-26%
rollup-base-private	0m19.305s	-27%
private-kernel-tail	0m0.677s	-11%
private-kernel-reset	0m1.518s	3%
private-kernel-inner	0m0.960s	-4%
parity-root	0m0.525s	-5%

[github-actions]

Compilation Sample

Program	Compilation Time	%
sha256_regression	0m1.581s	9%
regression_4709	0m0.876s	7%
ram_blowup_regression	0m15.580s	2%
rollup-base-public	2m8.482s	-39%
rollup-base-private	1m52.855s	-40%
private-kernel-tail	0m1.165s	0%
private-kernel-reset	0m8.488s	14%
private-kernel-inner	0m2.480s	-11%
parity-root	0m0.871s	-13%
noir-contracts	1m34.686s	-43%

[TomAFrench]

Chucking in the merge queue while I create issues.