[Valgrind-developers] Remove U256 type from VEX

[Florian K. <fl...@ei...>]

U256 is a 16-byte wide type defined as an array of UInts and therefore 
presumably meant to represent 256 bit wide integers.
There are, however, no IROps operating on or yielding such a value.
The type is only used to declare guest_YMM0 etc. in libvex_guest_amd64.h
But those registers are vector registers. Therefore, V256 is the proper type.

The reg-tested attached patch removes the U256 typedef and adds a V128 [2] 
variant to V256.

I realise that this is an incompatible change in the VEX API.
But given that U256 is effectively a dead type within VEX I was wondering 
whether we have to go through some kind of deprecation protocol.

OK?

   Florian