feat: avoid branch condition negations in ValueMerger

[TomAFrench]

Description

Problem*

Resolves

Summary*

This PR modifies how we merge values from pred * a + (1-pred) * b to b + pred * (a-b) which avoids another multiplication. This is being done manually in here so it's worth considering as the default merger strategy.

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.

[github-actions]

Changes to circuit sizes

Generated at commit: 9032919fe8d94d52b2471a4923f390fa70aa4709, compared to commit: fb1a8ca67c58d87991358078e6c532b49824fdb8

🧾 Summary (10% most significant diffs)

Program	ACIR opcodes (+/-)	%	Circuit size (+/-)	%
conditional_1	-1,202 ✅	-28.94%	-1,196 ✅	-10.03%
regression_5252	-3,600 ✅	-10.41%	-5,336 ✅	-11.13%
hashmap	-17,088 ✅	-26.64%	-26,078 ✅	-18.91%
debug_logs	-6 ✅	-13.04%	-54 ✅	-73.97%

---

Full diff report 👇

Program	ACIR opcodes (+/-)	%	Circuit size (+/-)	%
array_sort	55 (+2)	+3.77%	132 (+4)	+3.13%
sha256_regression	20,388 (-14,702)	-41.90%	200,488 (+3,028)	+1.53%
binary_operator_overloading	284 (+2)	+0.71%	4,465 (+2)	+0.04%
bench_eddsa_poseidon	16,441 (+2)	+0.01%	19,588 (+2)	+0.01%
eddsa	64,631 (+2)	+0.00%	65,817 (+2)	+0.00%
conditional_regression_short_circuit	366 (-4)	-1.08%	11,150 (-5)	-0.04%
array_dynamic	98 (-4)	-3.92%	3,710 (-4)	-0.11%
conditional_2	19 (-2)	-9.52%	2,778 (-3)	-0.11%
schnorr	1,510 (-9)	-0.59%	23,903 (-29)	-0.12%
array_if_cond_simple	102 (-6)	-5.56%	3,125 (-7)	-0.22%
regression_3607	34 (-5)	-12.82%	2,795 (-7)	-0.25%
regression_mem_op_predicate	47 (-9)	-16.07%	3,576 (-10)	-0.28%
conditional_regression_661	20 (-9)	-31.03%	2,787 (-9)	-0.32%
bigint	1,118 (-48)	-4.12%	8,076 (-41)	-0.51%
u128	654 (-26)	-3.82%	4,664 (-25)	-0.53%
nested_array_in_slice	853 (-30)	-3.40%	5,473 (-30)	-0.55%
regression	108 (-33)	-23.40%	3,647 (-36)	-0.98%
hash_to_field	540 (-42)	-7.22%	3,582 (-36)	-1.00%
ram_blowup_regression	162,771 (-10,752)	-6.20%	667,648 (-9,216)	-1.36%
slice_dynamic_index	916 (-66)	-6.72%	6,351 (-102)	-1.58%
sha256	1,542 (-336)	-17.89%	20,882 (-425)	-1.99%
slices	761 (-74)	-8.86%	3,859 (-88)	-2.23%
sha256_var_witness_const_regression	1,339 (-336)	-20.06%	16,791 (-425)	-2.47%
sha256_var_size_regression	9,041 (-7,120)	-44.06%	66,856 (-4,105)	-5.78%
conditional_1	2,952 (-1,202)	-28.94%	10,728 (-1,196)	-10.03%
regression_5252	30,996 (-3,600)	-10.41%	42,627 (-5,336)	-11.13%
hashmap	47,046 (-17,088)	-26.64%	111,860 (-26,078)	-18.91%
debug_logs	40 (-6)	-13.04%	19 (-54)	-73.97%

[TomAFrench]

@guipublic This could be something interesting to take up. I'm not able to devote proper time to it atm but it looks very promising in terms of gate count reductions. I'm not sure why the sha256_regression test has regressed though.