Add SSA-CFG invariant checks

[msooseth]

Adds invariant checks to SSA-CFG. Currently, the SSA-CFG fails the phis at the beginning, sol it's disabled. MainExit count is also often 0, even though I think it should be 1? Also disabled. Both are disabled with #if 0 so it's ugly as hell and should be quite visible. All is gated with SLOW_DEBUG, but this is ugly and will need to go. Look past it please.

Checks added are listed below.

InstructionStore:

• Per-opcode checks (e.g. Const has no inputs)
• Every referenced operand exists
• Literal dedup table is actually dedup-ed
• Value-dependency graph acyclic

SSACFG:

• Projections: no other instructions in between, returns match projection number
• Edge consistency between blocks
• Phi/Upsilon symmetries/relationships
• Every instruction is first defined, then used
• Every instruction scheduled

Things I need help with

• We need to figure out what to do with SLOW_DEBUG. Some of these should run always, and some only in a debug build. We should decide which ones.
• What should we do with _ctx? It's built, always, but only used when emitting an error, which could be expensive. Perhaps only pass non-empty _ctx in debug build only?
• The #if 0-s in SSACFG.cpp are things we want, but are not currently true. Should they stay?
• The SSACFG.h has a public&private block in SLOW_DEBUG, which makes it a bit messy. But also not sure how to structure it otherwise. Maybe a friend class? Perhaps would make more sense?

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
• Claude was used
• I did, actually, review, line by line, and character by character, the whole thing

[github-actions]

There was an error when running Code Style Check for commit 6f3a086b8c96e1a49eec9e168968a5af225d6e84:

Coding style error:
[Use angle brackets for includes] libyul/backends/evm/ssa/SSACFG.cpp:19:#include "libyul/Exceptions.h"
[Missing space after keyword] libyul/backends/evm/ssa/InstructionStore.cpp:62:	if(i.isTombstone())
[Missing space after keyword] libyul/backends/evm/ssa/SSACFG.cpp:247:			for(i2 = (ssize_t)i-1; i2 >= 0; i2--) {
[Opening brace on same line as if] libyul/backends/evm/ssa/SSACFG.cpp:250:				if (!inst.isProjection()) {
[const should be on the right side of the type] libyul/backends/evm/ssa/SSACFG.cpp:224:		parentBlock.forEachExit([&](const BlockId succBlockId){
[const should be on the right side of the type] libyul/backends/evm/ssa/SSACFG.cpp:223:		const auto& parentBlock = block(parentBlockId);
[Use tabs for indentation] libyul/backends/evm/ssa/InstructionStore.cpp:63:	  return;
[Use tabs for indentation] libyul/backends/evm/ssa/InstructionStore.h:106:		    return opcode == InstOpcode::Call || opcode == InstOpcode::BuiltinCall;

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