[PowerPC][AIX] Set the HasAlloca flag in the AIX Traceback Table only if R31...

[PowerPC][AIX] Set the HasAlloca flag in the AIX Traceback Table only if R31 is used as a frame pointer

After c0639464 usage of R31 doesn't necessarily mean
that alloca is used. The `TracebackTable::IsAllocaUsedMask` flag should be set only
when R31 is used as a frame pointer.

On AIX the `function calls alloca' bit seems to be set whenever R31 is
set up as a frame pointer, even when there is no alloca call.

Reviewed By: lkail

Differential Revision: https://reviews.llvm.org/D108141