[X86] Fix fptoui conversions

This fixes two issues in x86 fptoui lowering.
1) Makes conversions from f80 go through the right path on AVX-512.
2) Implements an inline sequence for fptoui i64 instead of a library
call. This improves performance by 6X on SSE3+ and 3X otherwise.
Incidentally, it also removes the use of ftol2 for fptoui, which was
wrong to begin with, as ftol2 converts to a signed i64, producing
wrong results for values >= 2^63.

Patch by: mitch.l.bodart@intel.com
Differential Revision: http://reviews.llvm.org/D11316

llvm-svn: 245924