Optimizer: Eliminate dead mstore instructions from memoryguard-derived struct allocations

[OminduD]

Description

Fixes #16927

Fixes a missed optimisation in the IR pipeline where accessing a single member of an abi.decode-d struct leaves several dead mstore instructions that the UnusedStoreEliminator should remove.

Reproducer:

struct OnlyUnvalidated { uint256 tokenId; uint256 b; uint256 c; uint256 d; }

contract C {
    function decodeUnvalidatedStruct(bytes calldata data) external pure returns (uint256) {
        return abi.decode(data, (OnlyUnvalidated)).tokenId;
    }
}

The IR optimizer generates a scratch region at _1 = memoryguard(0x80), writes all four struct members into it, and then — because LoadResolver already forwards the .tokenId value directly — the four mstores are dead. However, UnusedStoreEliminator was unable to remove them because KnowledgeBase::explore() did not recognise memoryguard(N) as the constant N. Without that knowledge, no non-overlap between the scratch slots (0x80–0xfe) and the return target (newFreePtr = 0x100) could be proved, so all four stores were conservatively kept.

Fix:

Teach KnowledgeBase::explore(Expression const&) to fold memoryguard(literal) to its argument value, exactly like a plain Literal node. memoryguard always takes a single literal operand (enforced by the builtin's literalArguments metadata), so this is always safe.

Changes:

• libyul/optimiser/KnowledgeBase.h — add std::optional<BuiltinHandle> m_memoryGuardBuiltinHandle and initialise it in both constructors (same pattern as m_addBuiltinHandle / m_subBuiltinHandle).
• libyul/optimiser/KnowledgeBase.cpp — add a new else if branch in explore(Expression const&) that returns VariableOffset{YulName{}, literal->value.value()} when the callee is memoryguard. Also adds the missing braces around the sub branch to keep the else if chain well-formed.
• test/libyul/yulOptimizerTests/unusedStoreEliminator/memoryguard_dead_store.yul — new regression test directly modelling the IR pattern from the bug report; asserts that all four dead stores are eliminated.

Checklist

• I have read the contributing guidelines and the review checklist
• I have personally reviewed, understood, and tested every change in this PR

AI Disclosure

• No AI tools were used

Antigravity (Claude Sonnet 4.6) was used to navigate the codebase, cross-reference the interaction between KnowledgeBase, UnusedStoreEliminator, and UnusedStoreBase, and to draft this PR description. All code changes were authored, reviewed, and tested by the contributor.