[CostModel][X86] Fixed vXi8 uniform shift costs.

The 'fast' costs should only work for shifts by uniform constants (uniform non-constant are lowered using the slow default implementation).

Logical shifts were not taking into account that we must mask the psrlw result, so the costs needed to be doubled.

Added missing AVX2/AVX512BW costs as well.

llvm-svn: 291391