[X86] Fix cmp order in probing BuildStackAlignAND

Due to reversed arguments, the loop start was almost always skipping the
whole loop, since FinalStackProbed is probably less than StackPtr for
large alignments. The intent was to skip the loop if the first sub on
StackPtr made it less than FinalStackProbed already, so flip it.

Reviewed By: serge-sans-paille

Differential Revision: https://reviews.llvm.org/D139756