[X86] Do not assume types are legal in getFauxShuffleMask

Summary:
Make sure we do not assert on value types not being
simple in getFauxShuffleMask when analysing operations
such as "v8i16 = truncate v8i24".

Reviewers: RKSimon

Reviewed By: RKSimon

Subscribers: hiraditya, llvm-commits

Tags: #llvm

Differential Revision: https://reviews.llvm.org/D77136