Exception on underflow in subtraction between two signed numbers

[pirapira]

Since #796 we are in the process of adding runtime overflow checks. At 3f833c9, solc does not add runtime checks for underflows during subtraction between two signed numbers. This can be seen as follows

$ solc/solc test.sol --opcodes --optimize | grep CALLDATA
<snip> PUSH1 0x24 CALLDATALOAD PUSH1 0x4 CALLDATALOAD SUB PUSH1 0x60 SWAP1 DUP2 MSTORE PUSH1 0x20 SWAP1 RETURN 

[pirapira]

Delegated back to #796.