Missing assembly rules: for, switch, function

[cgewecke]

Several assembly features appear to be missing from the official grammar and use keywords / probably need special parsing.

EDIT: grammar for them can be found at: https://github.com/solidityj/solidity-antlr4/blob/master/Solidity.g4

Links to relevant code in AsmParser.cpp:

• for loops
• switch statements
• function definitions