InstCombine: Can't fold a phi arg load into the phi if the load is from a swifterror address

`swifterror` addresses are only allowed as operands to load, store, and
calls.

The following transformation is not allowed. It would create a phi with a
`swifterror` address operand.

```
 %addr = alloca swifterror i8*
 br %cond, label %bb1, label %b22

 bb1:
   %val1 = load i8*, i8** %addr
   br exit

 bb2:
   %val2 = load i8*, i8** %addr
   br exit

 exit:
   %val = phi [%val1, %bb1] [%val2, %bb2]
```

=>

```
 %addr = alloca swifterror i8*
 br %cond, label %bb1, label %b22

 bb1:
   br exit

 bb2:
   br exit

 exit:
   %val_addr = phi [%addr, %bb1] [%addr, %bb2]
   %val2 = load i8*, i8** %val_addr
```

rdar://89865485

Differential Revision: https://reviews.llvm.org/D121217