compiler: integrate ir package into compiler package

The ir package has long lost its original purpose (which was doing some
analysis and optimization on the Go SSA directly). There is very little
of it left, which is best integrated directly in the compiler package to
avoid unnecessary abstraction.

compiler/calls.go

@@ -35,7 +35,7 @@ const (
 
 // createCall creates a new call to runtime.<fnName> with the given arguments.
 func (b *builder) createRuntimeCall(fnName string, args []llvm.Value, name string) llvm.Value {
-	fn := b.ir.Program.ImportedPackage("runtime").Members[fnName].(*ssa.Function)
+	fn := b.program.ImportedPackage("runtime").Members[fnName].(*ssa.Function)
 	llvmFn := b.getFunction(fn)
 	if llvmFn.IsNil() {
 		panic("trying to call non-existent function: " + fn.RelString(nil))