[X86] Prefer reduced width multiplication over pmulld on Silvermont

Summary:
Prefer expansions such as: pmullw,pmulhw,unpacklwd,unpackhwd over pmulld.
On Silvermont [source: Optimization Reference Manual]:
PMULLD has a throughput of 1/11 [instruction/cycles].
PMULHUW/PMULHW/PMULLW have a throughput of 1/2 [instruction/cycles].

Fixes pr31202.

Analysis of this issue was done by Fahana Aleen.

Reviewers: wmi, delena, mkuper

Subscribers: RKSimon, llvm-commits

Differential Revision: https://reviews.llvm.org/D27203

llvm-svn: 288844