Opportunity to compress common info table representation.

I've looked at a lot of GHC produced assembly recently and noticed that most info tables describing stacks have the form:

.align 8
	.long	SDjR_srt-(block_cHmk_info)+296
	.long	0
	.quad	6151
	.quad	4294967326

I haven't managed to dig fully into the description however some observations:

• I noticed that the second .long directive almost always ends up being zero.
• When figuring out what is what I realized the first quad (describing the pointers) is almost never fully used.
• The last entrie (closure type + ?), here 4294967326 also seems quite repetitive given the size reserved.

So I looked in detail at spectral/simple:

• There are 2012 info tables of this sort with all of them having a zero in the second long.
• We also reserve 8 byte for the stack layout. However only a single of these tables requires more than 4 byte.

The compiled module has a size of 276384 Bytes, with 16092 being redundant:

• 4 bytes for 0
• 4 bytes unused stack description
• times 2012 info tables.

That is an overhead of 5,8% which seems like quite a lot to me.

The questions where to put that information is a different one. But only looking at the data and not how it is used tagging the pointer to the SRT table seems like a possibility.

The info table description 4294967326 also appeared over 1k times. Maybe it's possible to come up with a more efficient encoding there as well.

I didn't give it much thought yet since I don't have the time to do anything about it in the near future. But putting it here in case anyone is interested or looks into this in the future.

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