stage2: Add code generation for Load instruction in LLVM backend

The following now works:
```
export fn _start() noreturn {
    var x: bool = true;
    var y: bool = x;
    exit();
}

fn exit() noreturn {
    unreachable;
}
```

src/llvm_backend.zig

@@ -312,6 +312,7 @@ pub const LLVMIRModule = struct {
                         .arg => try self.genArg(inst.castTag(.arg).?),
                         .alloc => try self.genAlloc(inst.castTag(.alloc).?),
                         .store => try self.genStore(inst.castTag(.store).?),
+                        .load => try self.genLoad(inst.castTag(.load).?),
                         .dbg_stmt => blk: {
                             // TODO: implement debug info
                             break :blk null;
@@ -396,6 +397,11 @@ pub const LLVMIRModule = struct {
         return null;
     }
 
+    fn genLoad(self: *LLVMIRModule, inst: *Inst.UnOp) !?*const llvm.ValueRef {
+        const ptr_val = try self.resolveInst(inst.operand);
+        return self.builder.buildLoad(ptr_val, "");
+    }
+
     fn genBreakpoint(self: *LLVMIRModule, inst: *Inst.NoOp) !?*const llvm.ValueRef {
         // TODO: Store this function somewhere such that we dont have to add it again
         const fn_type = llvm.TypeRef.functionType(llvm.voidType(), null, 0, false);

src/llvm_bindings.zig

@@ -125,6 +125,9 @@ pub const BuilderRef = opaque {
 
     pub const buildStore = LLVMBuildStore;
     extern fn LLVMBuildStore(*const BuilderRef, Val: *const ValueRef, Ptr: *const ValueRef) *const ValueRef;
+
+    pub const buildLoad = LLVMBuildLoad;
+    extern fn LLVMBuildLoad(*const BuilderRef, PointerVal: *const ValueRef, Name: [*:0]const u8) *const ValueRef;
 };
 
 pub const BasicBlockRef = opaque {