Make isAliased property for fixed-offset stack objects adjustable

Make isAliased property for fixed-offset stack objects adjustable

We used to assume that any fixed-offset stack object was not aliased. This
meant that no IR value could point to the memory contained in such an object.
This is a reasonable default, but is not a universally-correct
target-independent fact. For example, on PowerPC (both Darwin and non-Darwin),
some byval arguments are allocated at fixed offsets by the ABI. These, however,
certainly can be pointed to by IR values. This change moves the 'isAliased'
logic out of FixedStackPseudoSourceValue and into MFI, and allows the isAliased
property to be overridden for fixed-offset objects.

This will be used by an upcoming commit to the PowerPC backend to fix PR20280.

No functionality change intended (the behavior of
FixedStackPseudoSourceValue::isAliased has been made more conservative for
callers that don't pass an MFI object, but I don't see any in-tree callers that
do that).

git-svn-id: https://llvm.org/svn/llvm-project/llvm/trunk@215794 91177308-0d34-0410-b5e6-96231b3b80d8