[X86] Centalize the 'sse4' hack to a single place in X86TargetInfo::setFeatureEnabledImpl. NFCI

Instead of detecting the string in 2 places. Just swap the string
to 'sse4.1' or 'sse4.2' at the top of the function.

Prep work for a patch to switch the rest of this function to a
table based system. And I don't want to include 'sse4a' in the
table.