[X86] Restore combineSelect BLENDV sign-bit fold
The optimization that folds X86ISD::BLENDV away when the condition's
sign bit is statically known was inadvertently dropped during a prior
rebase conflict resolution. Its absence caused unrelated CHECK lines
in combine-sdiv.ll, vector-compress.ll, vector-idiv-{u,s}div-*.ll to
fail because main expects the post-optimization codegen.
Restores the block at llvm/lib/Target/X86/X86ISelLowering.cpp line
~49587 (after combineLogicBlendIntoConditionalNegate, before the
VSELECT/BLENDV shuffle handling), matching origin/main verbatim.
