[X86] Raise the latency for VectorImul from 4 to 5 in Skylake scheduler models

Based on uops.info these should have 5 cycle latency as they did on Haswell/Broadwell. I have no additional internal information from Intel.

This was also shown as a discrepancy in the spreadsheet that was sent with an early llvm-dev post about llvm-exegesis.
It also matches Agner Fog.

Differential Revision: https://reviews.llvm.org/D74357