[TTI] Add generic SK_Broadcast shuffle costs

I noticed while fixing PR39368 that we don't have generic shuffle costs for broadcast style shuffles.

This patch adds SK_BROADCAST handling, but exposes ARM/AARCH64 lack of handling of this type, which I've added a fix for at the same time.

Differential Revision: https://reviews.llvm.org/D53570

llvm-svn: 345253