[GlobalOpt] Support index types when hoisting constexprs

[AGindinson]

Avoids crashes for operations like tensor.dim / tensor.from_elements when hoisting constant expressions.

The only HoistableTypeInterface's that are feasible to instantiate are the existing ones, for general tensors and for scalar indices. tensor<index> types need explicit handling, as default utils for bitwidth assessment do not (and should not) assume the exact size of the index type.

The approach taken is to patch up the common RankedTensorType paths in HoistableTypeInterface and ConstExpr analyses. The conservative assumption of 64-bit indices is wrapped around the common helpers like IREE::Util::getRoundedPhysicalStorageSize and IREE::Util::getTypeBitWidth - and therefore, isolated from the rest of the codebase.

Instead of the 64-bit assumption, an alternative approach would be to access module-level data layout information from within ConstExprAnalysis. Achieving this for HoistableTypeInterface seems less clear.

[MaheshRavishankar]

It can be tricky handling index bits this way. There could be mismatches if the initializer runs on a machine where index is mapped to 32-bits, and the rest of it maps index to 64-bits (or vice-versa). I dont know off the top of my head what would happen today.

[AGindinson]

@MaheshRavishankar Ah, that's a great catch. Intuitively, it seems that my optimization is plain incorrect if the host indexes in 32-bit. I'll have a go at querying TargetInfo then.