Use getTargetConstant for BLENDI, and add a test to catch it.

Summary: This fixes a crasher introduced by r372338.

Reviewers: echristo, arsenm

Subscribers: jvesely, wdng, nhaehnle, hiraditya, llvm-commits

Tags: #llvm

Differential Revision: https://reviews.llvm.org/D67785

Tighten up the test case.

llvm-svn: 372366