[X86] Simplify the TTI code for getInterleavedMemoryOpCost around for AVX512BW. NFCI

Previously the lambda for AVX512 passed out a flag that indicated whether AVX512BW was required and that was checked against the AVX512BW subtarget flag outside.

This patch changes the interface to pass the AVX512BW subtarget bit in and return its value if we detect 16 or 8 bit types.

llvm-svn: 319919