[NFC][LoopVectorizer] Remove VF.isScalable() assertion from...

[NFC][LoopVectorizer] Remove VF.isScalable() assertion from collectInstsToScalarize and getInstructionCost

This patch removes the assertion when VF is scalable and replaces
getKnownMinValue() by getFixedValue(),  so it still guards the code against
scalable vector types.
The assertions were used to guarantee that getknownMinValue were not used for
scalable vectors.

Differential Revision: https://reviews.llvm.org/D106359