[msan] Heuristically instrument unknown intrinsics.

[msan] Heuristically instrument unknown intrinsics.

This changes adds shadow and origin propagation for unknown intrinsics
by examining the arguments and ModRef behaviour. For now, only 3 classes
of intrinsics are handled:
- those that look like simple SIMD store
- those that look like simple SIMD load
- those that don't have memory effects and look like arithmetic/logic/whatever
  operation on simple types.

git-svn-id: https://llvm.org/svn/llvm-project/llvm/trunk@170530 91177308-0d34-0410-b5e6-96231b3b80d8