[PowerPC] Fix wrong codegen when stack pointer has to realign performing dynalloc

Current powerpc backend generates wrong code sequence if stack pointer
has to realign if `-fstack-clash-protection` enabled. When probing
dynamic stack allocation, current `PREPARE_PROBED_ALLOCA` takes
`NegSizeReg` as input and returns
`FinalStackPtr`. `FinalStackPtr=StackPtr+ActualNegSize` is calculated
correctly, however code following `PREPARE_PROBED_ALLOCA` still uses
value of `NegSizeReg`, which does not contain `ActualNegSize` if
`MaxAlign > TargetAlign`, to calculate loop trip count and residual
number of bytes.

This patch is part of fix of
https://bugs.llvm.org/show_bug.cgi?id=46759.

Differential Revision: https://reviews.llvm.org/D84152

(cherry picked from commit c3f9697f)