[X86] Fix wrong treatment of multi-lane blends in BUILD_VECTORtoBlendMask()

This fixes two separate bugs:
1) The mask for the high lane was not set correctly. That fixes PR24532.
2) The transformation should bail out if it believes it involves more than
2 lanes, as it does not currently do anything sensible in this case.

Differential Revision: http://reviews.llvm.org/D13505

llvm-svn: 249669