[SLP]Improve final minbitwidth analysis attempt.

Added part for demanded bits analysis in the IsPotentiallyTruncated to
improve minbitwidth analysis final attempts.

Metric: size..text

Program                                                                                                                                                size..text
                                                                                                                                                       results     results0    diff
                                                                           test-suite :: MultiSource/Benchmarks/MiBench/telecomm-gsm/telecomm-gsm.test    43069.00    42973.00 -0.2%
                                                                                  test-suite :: MultiSource/Benchmarks/mediabench/gsm/toast/toast.test    43066.00    42970.00 -0.2%

Extra trunc instructions are emitted to operate with <32 x i8> instead
of <32 x i16>, will be removed in the next patches.

Reviewers: RKSimon

Reviewed By: RKSimon

Pull Request: https://github.com/llvm/llvm-project/pull/87786