fix(Compiler/Expr): replace caseInteger list-indexing fallback with equalsInteger chain

[Unisay]

Summary

Fixes the execution cost regression documented in #7691.

When compiling caseInteger in the default SumsOfProducts mode, the fallback used PlutusTx.List.!!, which built a linked list at runtime and indexed into it with a Z-combinator. This caused a 3-5x cost increase for unsafeFromBuiltinData on multi-constructor types.

This PR replaces the fallback with a lazy equalsInteger/ifThenElse chain in PIR.
Uses the (all dead. resTy) / (/\dead -> branch) encoding to avoid evaluating non-matching branches. No runtime allocation, no Y-combinator, no linked list.

How it works

The non-BuiltinCasing branch in Compiler/Expr.hs now generates PIR equivalent to:

ifThenElse {all dead. resTy} (equalsInteger scrut 0) (/\dead -> b0) (/\dead -> ifThenElse {all dead. resTy} (equalsInteger scrut 1) (/\dead -> b1) (/\dead -> b2) {resTy}) {resTy} 

Which erases to delay/force in UPLC, matching the pre-regression pattern.
The BuiltinCasing branch is unchanged (native case on the integer).

Budget comparison

data ABC = A Integer | B Integer | C Integer, same source compiled in both modes:

SumsOfProducts (default, the regressed mode)

Test	Metric	Baseline (if False)	This PR	Regressed (master)
decode A (index 0)	CPU	1,244,851	1,292,851	2,214,190
	Memory	4,662	4,962	9,964
decode C (index 2)	CPU	2,050,823	1,746,441	4,054,578
	Memory	7,468	6,366	17,974

Baseline is generated by bypassing caseInteger (if False in unsafeFromDataClause),
which falls back to the old tuple-pattern-matching TH code. The small remaining difference
(~4% for index 0) is from casePair continuation vs fst/snd let-bindings in the
pair extraction, which is inherent to the caseInteger TH code path.

BuiltinCasing (unaffected by this change)

Test	Metric	Budget
decode A (index 0)	CPU	512,582
	Memory	2,596
decode C (index 2)	CPU	677,790
	Memory	2,998

[github-actions]

Execution Budget Golden Diff

4b90cc2 (master) vs 3f451f2

output

This comment will get updated when changes are made.

[zliu41]

This needs to be backported to the version the Hydra team is using.

[SeungheonOh]

looks good to me