[X86] Use __builtin_convertvector to replace some of the avx512 truncate builtins.

As long as the destination type is a 256 or 128 bit vector with the same number of elements we can use __builtin_convertvector to directly generate trunc IR instruction which will be handled natively by the backend.

Differential Revision: https://reviews.llvm.org/D46742

llvm-svn: 332266