[PowerPC] Allow stack frames without parameter save area

The PPCFrameLowering::determineFrameLayout routine currently ensures
that every function that allocates a stack frame provides space for the
parameter save area (via PPCFrameLowering::getMinCallFrameSize).

This is actually not necessary.  There may be functions that never call
another routine but still allocate a frame; those do not require the
parameter save area.  In the future, with the ELFv2 ABI, even some
routines that do call other functions do not need to allocate the
parameter save area.

While it is not a bug to allocate the parameter area when it is not
needed, it is better to avoid it to save stack space.

Note that when any particular function call requires the parameter save
area, this space will already have been included by ABI code in the size
the CALLSEQ_START insn is annotated with, and therefore included in the
size returned by MFI->getMaxCallFrameSize().

This means that determineFrameLayout simply does not need to care about
the parameter save area.  (It still needs to ensure that every frame
provides the linkage area.)  This is implemented by this patch.

Note that this exposed a bug in the new fast-isel code where the parameter
area was *not* included in the CALLSEQ_START size; this is also fixed.

A couple of test cases needed to be adapted for the new (smaller) stack
frame size those tests now see.