[X86] Mark ISD::FP_TO_UINT v16i8/v16i16 as Promote under AVX512 instead of...

[X86] Mark ISD::FP_TO_UINT v16i8/v16i16 as Promote under AVX512 instead of legal. Fix infinite loop in op legalization when promotion requires 2 steps.

Previously we had an isel pattern to add the truncate. Instead use Promote to add the truncate to the DAG before isel.

The Promote legalization code had to be updated to prevent an infinite loop if promotion took multiple steps because it wasn't remembering the previously tried value.

llvm-svn: 319259