[Thumb1] Fix and re-enable the load/store optimization pass

[Moritz Roth]

Hi Renato,

>> 0002: Make the merging more conservative on Thumb1.
>
> I just don't get this one. This should be disabled in Thumb1, right?
> Why bother changing the behaviour and not re-enabling it on the same
> patch?

Fair point, I've merged the last two patches as you suggested.

> Did you change getMaxInlineSizeThreshold() because you're not doing
> the LDM on two registers on T1 any more? You plan on doing the full
> fold in the future even on T1?
>
> Either way, a comment / fixme to that effect would be good.

Yes, the intention was to avoid generating long chains of loads/stores since
the algorithm in its current form can't properly merge all of them.
However, upon looking at some generated code again I've decided to scrap this.
Even with larger memcpys approaching the cut-off there is still a good amount
of LDM/STM folding going on. And yes, I'm planning on submitting a follow-up
patch to allow memcpys to be fully folded even on T1.

>> 0003: Re-enable the pass, and add test cases for the previous two patches.
>> There are three new regression tests created from simple test programs that
>> were miscompiled previously, and I’ve added –verify-machineinstrs to each of
>> the tests as well.
>
> If I got it right, you shouldn't have SUBS after some LDMs, so would
> be good to CHECK-NOT them on the appropriate tests.

Yes, that's right. I've made sure the tests where the pass would previously
insert a SUBS have such a check now. Updated patches are attached.

Cheers
Moritz

> Thanks!
> --renato
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