[Hexagon] Optimize post-increment load and stores in loops. (#82011)

This patch optimizes the post-increment instructions so that we can
packetize them together.
v1 = phi(v0, v3')
v2,v3  = post_load v1, 4
v2',v3'= post_load v3, 4

This can be optimized in two ways

v1 = phi(v0, v3')
v2,v3' = post_load v1, 8
v2' = load v1, 4