[X86] Make X86TTIImpl::getCastInstrCost properly handle the case where AVX512...

[X86] Make X86TTIImpl::getCastInstrCost properly handle the case where AVX512 is enabled, but 512-bit vectors aren't legal.

Unlike most cost model functions this code makes a lot of table lookups without using the results from getTypeLegalizationCost. This means 512-bit vectors can be looked up even when the type isn't legal.

This patch adds a check around the two tables that contain 512-bit types to make sure that neither of the types would be split by type legalization. Meaning 512 bit types are illegal. I wanted to write this in a somewhat generic way that uses type legalization query hooks. But if prefered, I can switch to just using is512BitVector and the subtarget feature.

Differential Revision: https://reviews.llvm.org/D54984

llvm-svn: 347786