Update base for Update on "[compiler] Optimize emission in normal (no…

…n-value) blocks"

In #29863 I tried to find a clean way to share code for emitting instructions between value blocks and regular blocks. The catch is that value blocks have special meaning for their final instruction — that's the value of the block — so reordering can't change the last instruction. However, in finding a clean way to share code for these two categories of code, i also inadvertently reduced the effectiveness of the optimization.

This PR updates to use different strategies for these two kinds of blocks: value blocks use the code from #29863 where we first emit all non-reorderable instructions in their original order, _then_ try to emit reorderable values. The reason this is suboptimal, though, is that we want to move instructions closer to their dependencies so that they can invalidate (merge) together. Emitting the reorderable values last prevents this.

So for normal blocks, we now emit terminal operands first. This will invariably cause _some_ of the non-reorderable instructions to be emitted, but it will intersperse reoderable instructions in between, right after their dependencies. This maximizes our ability to merge scopes.

I think the complexity cost of two strategies is worth the benefit, though i still need to double-check all the output changes.

[ghstack-poisoned]