[PowerPC] Convert more tests to opaque pointers (NFC)

* Add -fast-isel=false to func-alias.ll. The test was added as a
SelectionDAG test. Without this option, FastISel successfully selects
the call that had a ConstantExpr argument.
* fast-isel-branch.ll couldn't be handled by FastISel. Now it can,
hence the change in the stack offsets.