[X86] Don't compare i128 through vector if construction not cheap (PR41971)

Fix for https://bugs.llvm.org/show_bug.cgi?id=41971. Make the
combineVectorSizedSetCCEquality() transform more conservative by
checking that the bitcast to the vector type will be cheap/free
for both operands. I'm considering it cheap if it's a constant,
a load or already a vector. I've dropped the explicit check for
f128 because it should fall out naturally (in the cases where
it'd be detrimental).

Differential Revision: https://reviews.llvm.org/D62220

llvm-svn: 361352