[InstCombine] remove guard for generating a vector select

This is effectively NFC because we already do this transform after r175380:
http://reviews.llvm.org/rL175380

and also via foldBoolSextMaskToSelect().

This change should just make it a bit more efficient to match the pattern. 
The original guard was added in r95058:
http://reviews.llvm.org/rL95058

A sampling of codegen for current in-tree targets shows no problems. This
makes sense given that we're already producing the vector selects via the
other transforms.

llvm-svn: 271554