Look throught G_PTRTOINT and G_PTR_ADD nodes when looking for constant
offset for buffer stores. This also helps with merging of these instructions
later on.
Differential Revision: https://reviews.llvm.org/D95242
G_SHUFFLE_VECTOR is legal since it theoretically may help match op_sel
for VOP3P instructions. Expand it in some other way in case it doesn't
fold into the use instructions.
