[X86] Use vector widening to support sign extend from i1 when the dest type is not...

[X86] Use vector widening to support sign extend from i1 when the dest type is not 512-bits and vlx is not enabled.

Previously we used a wider element type and truncated. But its more efficient to keep the element type and drop unused elements.

If BWI isn't supported and we have a i16 or i8 type, we'll extend it to be i32 and still use a truncate.

git-svn-id: https://llvm.org/svn/llvm-project/llvm/trunk@319740 91177308-0d34-0410-b5e6-96231b3b80d8