basic_block: Add proxy member functions for the instruction list

Currently basic block kind of acts like a 'dumb struct' which makes things
a little more verbose to write (as opposed to keeping it all in one place,
I guess). It's also a little wonky conceptually, considering a block is
composed of instructions (i.e. 'contains' them).

So providing accessors that make it act more like a container can make working
with algorithms a little nicer. It also makes the API a little more
defined.

Ideally, the list would be only available through a function, but
currently, the pool allocator is exposed, which seems somewhat odd,
considering the block itself should manage its overall allocations
(with placement new, and regular new), rather than putting that
sanitizing directly on the IR emitter (it should just care about emission,
not block state). However, recontaining that can be followed up with,
as it's very trivial to do.

src/frontend/ir/basic_block.cpp

@@ -57,7 +57,7 @@ std::string DumpBlock(const IR::Block& block) {
         }
     };
 
-    for (auto inst = block.instructions.begin(); inst != block.instructions.end(); ++inst) {
+    for (auto inst = block.begin(); inst != block.end(); ++inst) {
         const Opcode op = inst->GetOpcode();
 
         if (GetTypeOf(op) != Type::Void) {