[PATCH] Teach rematerialiser more about subregs

[Tim Northover]

Hi Jakob,

> I think you probably need to make sure that the DefMI isn’t an partial register update:
>
>   %vreg1:sub_32<def> = MOVZwii whatever

Ah, yes. That check was already present (and I left it in place). It's
more obvious in the straight source than a messed-around diff.

> Are these checks necessary? It looks like CoalescerPair::setRegisters() will never return true if these checks are not satisfied.

After taking a deeper look at how that function works I think you're right.

> Otherwise, looks good!

Thanks. I've made the adjustments you suggested and committed it as r182872.

Tim.