[SVE][IR] Scalable Vector IR Type

* Adds a 'scalable' flag to VectorType
* Adds an 'ElementCount' class to VectorType to pass (possibly scalable) vector lengths, with overloaded operators.
* Modifies existing helper functions to use ElementCount
* Adds support for serializing/deserializing to/from both textual and bitcode IR formats
* Extends the verifier to reject global variables of scalable types
* Updates documentation

See the latest version of the RFC here: http://lists.llvm.org/pipermail/llvm-dev/2018-July/124396.html

Reviewers: rengolin, lattner, echristo, chandlerc, hfinkel, rkruppe, samparker, SjoerdMeijer, greened, sebpop

Reviewed By: hfinkel, sebpop

Differential Revision: https://reviews.llvm.org/D32530

llvm-svn: 361953