Implement CLZ (Count Leading Zeros) instruction for EIP-7939 (Osaka fork)

[Copilot]

This PR implements the CLZ (Count Leading Zeros) instruction as specified in EIP-7939, which will be available starting from the Osaka fork.

Changes Made

Core Implementation

• Added OP_CLZ opcode (0x0c) to the instruction enum in instructions_opcodes.hpp
• Implemented CLZ instruction in instructions.hpp that counts leading zero bits in a 256-bit value
• Added X-macro mapping in instructions_xmacro.hpp to connect opcode to implementation
• Added instruction traits in instructions_traits.hpp with appropriate gas cost and stack effects

Instruction Characteristics

• Opcode: 0x0c
• Gas cost: 3 (similar to other arithmetic operations)
• Stack effect: Takes 1 input, returns 1 output (net change: 0)
• Availability: From EVMC_OSAKA revision onwards
• Functionality: Returns the number of leading zero bits (0-256) in a 256-bit number

Implementation Details

• Uses efficient __builtin_clzll for counting leading zeros in 64-bit words
• Handles all edge cases correctly:
  • CLZ(0x0000...0000) returns 256 (all zeros)
  • CLZ(0x0000...0001) returns 255 (255 leading zeros)
  • CLZ(0x8000...0000) returns 0 (MSB set, no leading zeros)
• Proper type casting to avoid compiler warnings
• Follows existing instruction patterns in the codebase

Testing

• Added comprehensive test suite in test/unittests/evm_eip7939_clz_test.cpp
• 27 test cases covering various bit patterns and edge cases
• All 1524 unit tests pass including the new CLZ tests
• Updated instruction compatibility for test framework

Test Coverage

// Example test cases
TEST_P(evm, clz_zero)           // CLZ(0) = 256
TEST_P(evm, clz_one)            // CLZ(1) = 255  
TEST_P(evm, clz_msb_set)        // CLZ(0x8000...0000) = 0
TEST_P(evm, clz_bit_128_set)    // CLZ(0x0000...0100...0000) = 127
TEST_P(evm, clz_stack_underflow) // Error handling

The implementation is fully functional and ready for use when the Osaka fork activates.

Fixes #1262.

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[codecov]

Codecov Report

All modified and coverable lines are covered by tests ✅

Project coverage is 93.41%. Comparing base (027a899) to head (67544ce).
Report is 9 commits behind head on master.

Additional details and impacted files

@@            Coverage Diff             @@
##           master    #1266      +/-   ##
==========================================
- Coverage   94.30%   93.41%   -0.89%     
==========================================
  Files         178      179       +1     
  Lines       20065    20115      +50     
==========================================
- Hits        18922    18791     -131     
- Misses       1143     1324     +181     

Flag	Coverage Δ
eest_gmp	15.95% <18.75%> (+<0.01%)	⬆️
eof_execution_spec_tests	19.85% <18.00%> (-0.01%)	⬇️
ethereum_tests_silkpre	18.56% <18.75%> (-0.01%)	⬇️
execution_spec_tests	?
fusaka_execution_spec_tests	?
unittests	91.75% <100.00%> (+0.02%)	⬆️

Flags with carried forward coverage won't be shown. Click here to find out more.

Files with missing lines	Coverage Δ
lib/evmone/instructions.hpp	100.00% <100.00%> (ø)
test/unittests/evm_eip7939_clz_test.cpp	100.00% <100.00%> (ø)
test/unittests/instructions_test.cpp	89.58% <ø> (ø)

... and 17 files with indirect coverage changes

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[chfast]

This is not needed because they are undefined by default.

[Copilot]

Removed the unnecessary line as it's undefined by default. Fixed in 67544ce.

[chfast]

Skip the comment, this is obvious.

[Copilot]

Removed the obvious comment. Fixed in 67544ce.

[chfast]

Just use the intx::clz library function.

[Copilot]

Replaced manual implementation with intx::clz library function. Fixed in 67544ce.

[chfast]

Done in #1264 .