[X86] Add exhaustive cost model testing for sext/zext for all vector types we...

[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.

llvm-svn: 347719