[X86][AVX] Fixed issue where a long chain of shuffles could attempt to combine...

[X86][AVX] Fixed issue where a long chain of shuffles could attempt to combine to a single (illegal) PSHUFB instruction.

Its not enough that we test for SSSE3 - that's only OK for 128-bit vectors - we also need to test for AVX2 / AVX512BW for 256/512 bit vector cases.

llvm-svn: 263239