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

[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.

git-svn-id: https://llvm.org/svn/llvm-project/llvm/trunk@260296 91177308-0d34-0410-b5e6-96231b3b80d8