[msan] Improve handling of Intrinsic::is_fpclass after c55fffe

[msan] Improve handling of Intrinsic::is_fpclass after c55fffe

c55fffe replaced fcmp with fpclass.

```
declare i1 @llvm.is.fpclass(<fptype> <op>, i32 <test>)
declare <N x i1> @llvm.is.fpclass(<vector-fptype> <op>, i32 <test>)
```

Perfect fix will require checking bits of <op> corresponding to <test>
argument. For now just propagate shadow without reporting before
intrinsic. Still existing handling of fcmp is also simple OR, so it's
not making it worse.

Reviewed By: eugenis

Differential Revision: https://reviews.llvm.org/D149491