[X86][AVX-512] Don't raise inexact in ceil, floor, round, trunc.

The non-AVX-512 behavior was changed in r248266 to match N1778
(C bindings for IEEE-754 (2008)), which defined the four functions
to not raise the inexact exception ("rint" is still defined as raising
it).

Update the AVX-512 lowering of these functions to match that: it should
not be different.

llvm-svn: 306299