[PowerPC] Make sure the first probe is full size or is the last probe when stack is realigned

When `-fstack-clash-protection` is enabled and stack has to be realigned, some parts of redzone is written prior the probe, so probe might overwrite content already written in redzone. To avoid it, we have to make sure the first probe is at full probe size or is the last probe so that we can skip redzone.

It also fixes violation of ABI under PPC where `r1` isn't updated atomically.

This fixes https://bugs.llvm.org/show_bug.cgi?id=49903.

Reviewed By: jsji

Differential Revision: https://reviews.llvm.org/D100290