[X86] Add exhaustive cost model testing for sext/zext for all vector types we reasonably support. Add cost model tests for truncating to vXi1.
Our sext/zext cost modeling was somewhat incomplete. And had no coverage for the fact that avx512bw v32i16/v64i8 types return a scalarization cost.
Truncates are a whole different mess because isTruncateFree is returning true for vectors when it shouldn't and that's the fall back for anything not in the tables.
git-svn-id: https://llvm.org/svn/llvm-project/llvm/trunk@347719
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