[x86] Fix an embarressing bug in the INSERTPS formation code. The mask

computation was totally wrong, but somehow it didn't really show up with
llc.

I've added an assert that triggers on multiple existing test cases and
updated one of them to show the correct value.

There appear to still be more bugs lurking around insertps's mask. =/
However, note that this only really impacts the new vector shuffle
lowering.

llvm-svn: 217289