[Verifier] Add checks for masked.load and masked.store intrinsics

While trying to track down what appears to be a LoopVectorizer bug, I noticed that we had no validation of the correctness of calls emitted to @llvm.masked.load and @llvm.masked.store.  This meant malformed IR was showing up much much later than it should.  Hopefully, having Verifier rules in place will make this easier to isolate.

llvm-svn: 260296