Scalable vector type

[my4ng]

Scalable vector was added to LLVM 11, to support vector extensions with compile-time-unknown vector length such as arm's SVE and RISC-V's RVV. llvm_sys has the LLVMScalableVectorType, but inkwell currently has no support for it.

~~I am currently working on a solution that reuses the VectorType to support both fixed and scalable kinds, in a non-breaking manner.~~

EDIT: A separate scalable vector type and value are required to prevent mixing them with the fixed kind.