[X86] Fix AvoidStoreForwardingBlocks pass for negative displacements

Fixes https://bugs.llvm.org/show_bug.cgi?id=39926.

The size of the first copy was computed as
std::abs(std::abs(LdDisp2) - std::abs(LdDisp1)), which results in
skipped bytes if the signs of LdDisp2 and LdDisp1 differ. As far as
I can see, this should just be LdDisp2 - LdDisp1. The case where
LdDisp1 > LdDisp2 is already handled in the code above, in which case
LdDisp2 is set to LdDisp1 and this subtraction will evaluate to
Size1 = 0, which is the correct value to skip an overlapping copy.

Differential Revision: https://reviews.llvm.org/D55485

llvm-svn: 348750