[hwasan] Do not use short granule tags as poison tags.

Short granule tags as poison cause a UaF to read the referenced
memory to retrieve the tag, and means we do not detect the UaF
if the last granule's tag is still around.

This only increases the change of not catching a UaF from
0.39 % (1 / 256) to 0.42 % (1 / (256 - 17)).

Reviewed By: eugenis

Differential Revision: https://reviews.llvm.org/D104304