[IR] move shuffle mask queries from TTI to ShuffleVectorInst

The optimizer is getting smarter (eg, D47986) about differentiating shuffles 
based on its mask values, so we should make queries on the mask constant 
operand generally available to avoid code duplication.

We'll probably use this soon in the vectorizers and instcombine (D48023 and 
https://bugs.llvm.org/show_bug.cgi?id=37806).

We might clean up TTI a bit more once all of its current 'SK_*' options are 
covered.

Differential Revision: https://reviews.llvm.org/D48236

llvm-svn: 335067