implement codegen of calls. We can now compile:

double foo( int (*FP)(int, int, int)) {
  return FP(1, 2, 3);
}

to:

define double @foo(i32 (i32, i32, i32)* %FP) {
entry:
        %call = tail call i32 %FP( i32 1, i32 2, i32 3 )
        %conv = sitofp i32 %call to double
        ret double %conv
}

llvm-svn: 39658