Separate the check for blend shuffle_vector masks

Summary:
Separate the check for blend shuffle_vector masks into isBlendMask.
This function will also be used to check if a vector shuffle is legal. No
change in functionality was intended, but we ended up improving codegen on
two tests, which were being (more) optimized only if the resulting shuffle
was legal.

Reviewers: nadav, delena, andreadb

Subscribers: llvm-commits

Differential Revision: http://reviews.llvm.org/D3964