[SimplifyCFG] Update workaround for PR30188 to also include loads

I should have realised this the first time around, but if we're avoiding sinking stores where the operands come from allocas so they don't create selects, we also have to do the same for loads because SROA will be just as defective looking at loads of selected addresses as stores.

Fixes PR30188 (again).

llvm-svn: 280792