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