[X86][CostModel] Don't lookup intrinsic cost tables if the intrinsic isn't one we...

[X86][CostModel] Don't lookup intrinsic cost tables if the intrinsic isn't one we care about

We're seeing some issues internally where we sent some intrinsics into the cost model that the getTypeLegalizationCost call fails on, but X86 specific tables don't care about. Our base class implementation takes care of them. We'd just like X86 backend to ignore them.

This patch makes sure the switch returned something X86 cares about and skips the table lookups and type legalization call if not. Probably more efficient too since we don't go scanning the tables for every intrinsic we could possibly see.

Differential Revision: https://reviews.llvm.org/D54711

llvm-svn: 347248