[X86] Fix SLM scheduler model for PMULLD (PR37059)

Adjust the PMULLD entry to match the Intel AoM numbers - PMULLD is a uop nightmare on SLM and we should model it as such.

We had reports of internal regressions the last time this was attempted (rG13a0f83a05ff), but no public repros, and tests I did last year when I had access to a SLM box failed to see anything. My hunch is that the more aggressive PMULLD -> PMADDWD folds we now perform might have helped. We can revisit this again if we ever receive an actual repro.

Fixes #36407