Split parse nodes out from instructions because they're rarely used. (#3590)

The parse nodes are still tracked as part of the same value store
interface in order to ensure parity, but they're split out from Inst
itself in order to reduce the size of Inst -- the expectation is that
they don't need to be passed around quite as much.

This change doesn't actually reduce the passing very much, although
there are hints of it: AddInstAndPush doesn't typically need a separate
parse node from the one on the Inst itself, for example. In a couple
spots I changed code to rely a little more on the InstId until the
ParseNode is needed, but it's very low hanging fruit where done. I think
convert could do more to not eagerly fetch the parse node before its
use, but more cleanup felt it would be easier to handle separately. I'm
currently viewing this as making such cleanup _possible_ rather than
executing on it up-front.

But also, I want to make sure there's a consensus to head in this
direction before pulling the trigger. We speculated that this would
result in the parse node being passed around less, and I do think that's
the case, although it's a bit fuzzy in the change.

---------

Co-authored-by: Richard Smith <richard@metafoo.co.uk>

toolchain/check/handle_expr_statement.cpp

@@ -15,7 +15,7 @@ static auto HandleDiscardedExpr(Context& context, SemIR::InstId expr_id)
   // If we discard an initializing expression, convert it to a value or
   // reference so that it has something to initialize.
   auto expr = context.insts().Get(expr_id);
-  Convert(context, expr.parse_node(), expr_id,
+  Convert(context, context.insts().GetParseNode(expr_id), expr_id,
           {.kind = ConversionTarget::Discarded, .type_id = expr.type_id()});
 
   // TODO: This will eventually need to do some "do not discard" analysis.