[X86] Don't exit from foldOffsetIntoAddress if the Offset is 0, but AM.Disp is non-zero.

This is an alternate fix for the issue D73606 was trying to
solve.

The main issue here is that we bailed out of
foldOffsetIntoAddress if Offset is 0. But if we just found a
symbolic displacement and AM.Disp became non-zero
earlier, we still need to validate that AM.Disp with the symbolic
displacement.

This passes fold-add-pcrel.ll.

Differential Revision: https://reviews.llvm.org/D73608