[X86] Revert part of r332266: Use __builtin_convertvector to replace some of...

[X86] Revert part of r332266: Use __builtin_convertvector to replace some of the avx512 truncate builtins.

The masking doesn't work right in the backend for the ones that produce byte or word elements without avx512bw.

llvm-svn: 332322