[x86] Move the vector popcount tests into non-ISA files, and instead

organize them by the width of vector.

This makes it a lot easier to see that we're covering all of the vector
types but not doing so excessively. This also adds tests across the
spectrum of SSE versions in addition to the AVX versions.

If you're really tired of seeing the *massive* sprawl of scalarized code
for this, don't worry, I'm just about to land Bruno's patch that
dramatically improve the situation for SSSE3 and newer.

llvm-svn: 238520