[SLP]Do not require external uses for roots and single use for other...

[SLP]Do not require external uses for roots and single use for other instructions in computeMinimumValueSizes. (#72679)

After changes, that does not require support from InstCombine, we can
drop some extra requirements for values-to-be-demoted. No need to check
for external uses for roots/other instructions, just check that the
  no non-vectorized insertelement instruction, which may require
  widening.

Review: https://github.com/llvm/llvm-project/pull/72679