[IR] Support scalable vectors in ShuffleVectorInst::increasesLength

Since the length of the llvm::SmallVector shufflemask is related to the
minimum number of elements in a scalable vector, it is fine to just get
the Min field of the ElementCount. This is already done for the similar
function changesLength, tests have been added for both.

Reviewed By: sdesmalen

Differential Revision: https://reviews.llvm.org/D92472