Recommit "[LAA] Support pointer phis in loop by analyzing each incoming pointer."

SCEV does not look through non-header PHIs inside the loop. Such phis
can be analyzed by adding separate accesses for each incoming pointer
value.

This results in 2 more loops vectorized in SPEC2000/186.crafty and
avoids regressions when sinking instructions before vectorizing.

Fixes PR50296, PR50288.

Reviewed By: Meinersbur

Differential Revision: https://reviews.llvm.org/D102266