LSV was using two vector sets (heads and tails) to track pairs of adjiacent position to vectorize.

A recent optimization is trying to obtain the longest chain to vectorize and assumes the positions

in heads(H) and tails(T) match, which is not the case is there are multiple tails for the same head.

e.g.:

i1: store a[0]

i2: store a[1]

i3: store a[1]

Leads to:

H: i1

T: i2 i3

Instead of:

H: i1 i1

T: i2 i3

So the positions for instructions that follow i3 will have different indexes in H/T.

This patch resolves PR29148.

This issue also surfaced the fact that if the chain is too long, and TLI

returns a "not-fast" answer, the whole chain will be abandoned for

vectorization, even though a smaller one would be beneficial.

Added a testcase and FIXME for this.