memcheck powerpc subfe x, x, x initializes x to 0 or -1 based on CA

memcheck powerpc subfe x, x, x initializes x to 0 or -1 based on CA

GCC might use subfe x, x, x to initialize x to 0 or -1, based on
whether the carry flag is set. This happens in some cases when g++
compiles resetting a unique_ptr. The "trick" used by the compiler is
that it can AND a pointer with the register x (now 0x0 or 0xffffffff)
to set something to NULL or to the given pointer.

subfe is implemented as rD = (log not)rA + rB + XER[CA]
if we instead implement it as rD = rB - rA - (XER[CA] ^ 1)
then memcheck can see that rB and Ra cancel each other out if they
are the same.

https://bugs.kde.org/show_bug.cgi?id=404054