Compilers construction 2nd assignment

Compiler of simple imperative language for llvm

Optimizations:
1. constants propagation
2. copy propagation
3. expressions folding
4. local common subexpression elimination
4a. there's a variant of simple global common subexpression elimination but
at this point it's inefficient and doesn't handle cycles in flow graph very well
5. some peepholes optimizations

For instance:

    string a = "aaa";
    string b = "aaa";
    string c = a + "bb";
    string d = b + "bb";
    if (c == d) {
        printString(a + b);
    }
    if (a + "bb" == "aaa" + "bb") {
        printString(b + a);
    }
    return 0;

results in the following llvm code
define i32 @main() {
    %t1 = getelementptr [4 x i8], [4 x i8]* @.str.0, i32 0, i32 0
    %t2 = getelementptr [3 x i8], [3 x i8]* @.str.1, i32 0, i32 0
    %t3 = call i8* @__concat(i8* %t1, i8* %t2)
    %t6 = call i8* @__concat(i8* %t1, i8* %t1)
    call void @printString(i8* %t6)
    call void @printString(i8* %t6)
    ret i32 0
}

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Compiled with ghc 8.6.4
Uses llvm-as and llvm-link (6.0)