Add an option to the shuffle fuzzer that lets you fuzz exclusively

within a single bit-width of vectors. This is particularly useful for
when you know you have bugs in a certain area and want to find simpler
test cases than those produced by an open-ended fuzzing that ends up
legalizing the vector in addition to shuffling it.

git-svn-id: https://llvm.org/svn/llvm-project/llvm/trunk@215056 91177308-0d34-0410-b5e6-96231b3b80d8