[shuffle] Teach the shufflevector fuzzer to support fixed element types.

I'm using this to try to find more minimal test cases by re-fuzzing
within a specific domain once errors are found.

git-svn-id: https://llvm.org/svn/llvm-project/llvm/trunk@215823 91177308-0d34-0410-b5e6-96231b3b80d8
[x86] Fix an indentation goof in a prior commit. Should have re-run
clang-format.

git-svn-id: https://llvm.org/svn/llvm-project/llvm/trunk@215824 91177308-0d34-0410-b5e6-96231b3b80d8