[X86 Codegen] Fixed a bug in unsigned saturation

PACKUSWB converts Signed word to Unsigned byte, (the same about DW) and it can't be used for umin+truncate pattern.
AVX-512 VPMOVUS* instructions fit the pattern since they convert Unsigned to Unsigned.

See https://llvm.org/bugs/show_bug.cgi?id=31773

Differential Revision: https://reviews.llvm.org/D29196

llvm-svn: 293431