[PowerPC] Fix wrong codegen when stack pointer has to realign in prologue
Current powerpc backend generates wrong code sequence if stack pointer has to realign if -fstack-clash-protection enabled. When probing in prologue, backend should generate a subtraction instruction rather than a `stux` instruction to realign the stack pointer. This patch is part of fix of https://bugs.llvm.org/show_bug.cgi?id=46759. Differential Revision: https://reviews.llvm.org/D84218
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