[SVE] Fix ubsan issues in DecodeIITType

In an earlier patch I removed the need for
IITDescriptor::ScalableVecArgument, which involved changing
DecodeIITType to pull out the last IIT_Info from the list. However,
it turns out this is unsafe and causes ubsan failures. I've tried to
fix this a different way by simply passing the last IIT_Info as an
additional argument to DecodeIITType.

Differential Revision: https://reviews.llvm.org/D81057