[PowerPC] Mark fixed-offset byvals as pointed-to by IR values

[PowerPC] Mark fixed-offset byvals as pointed-to by IR values

A byval object, even if allocated at a fixed offset (prescribed by the ABI) is
pointed to by IR values. Most fixed-offset stack objects are not pointed-to by
IR values, so the default is to assume this is not possible. However, we need
to override the default in this case (instruction scheduling can cause
miscompiles otherwise).

Fixes PR20280.

git-svn-id: https://llvm.org/svn/llvm-project/llvm/trunk@215795 91177308-0d34-0410-b5e6-96231b3b80d8