[X86][SSE] combineTruncateWithSat - use truncateVectorWithPACK down to 64-bit subvectors

Add support for chaining PACKSS/PACKUS down to 64-bit vectors by using only a single 128-bit input.

git-svn-id: https://llvm.org/svn/llvm-project/llvm/trunk@325494 91177308-0d34-0410-b5e6-96231b3b80d8
3 files changed