The length of a scalable vector is unknown during compilation. When
vectorising loops the runtime length is required to update induction
variables and thus a representation is required within the IR.
This patch introduces the 'vscale' identifier to represent the scaling
factor 'n' of a scalable vector of the form '<n x #elements x ty>'.
In use, induction variable updates for scalable vectorisation become:
; i += number_of_elements(<n x 4 x i32) %i.next = add i32 %i, mul (i32 vscale, i32 4)
Picking up where Paul left off with https://reviews.llvm.org/D27103