[LoopVectorize] In LoopVectorize.cpp start using getSymbolicMaxBackedgeTakenCount (#108833)

LoopVectorizationLegality currently only treats a loop as legal to vectorise
if PredicatedScalarEvolution

[LoopVectorize] In LoopVectorize.cpp start using getSymbolicMaxBackedgeTakenCount (#108833)

LoopVectorizationLegality currently only treats a loop as legal to vectorise
if PredicatedScalarEvolution::getBackedgeTakenCount returns a valid
SCEV, or more precisely that the loop must have an exact backedge taken
count. Therefore, in LoopVectorize.cpp we can safely replace all calls to
getBackedgeTakenCount with calls to getSymbolicMaxBackedgeTakenCount,
since the result is the same.

This also helps prepare the loop vectoriser for PR #88385.

show more ...