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[BasicTTIImpl] Fix getCastInstrCost for scalable vectors by querying for…

Authored by sdesmalen on Feb 12 2021, 12:00 AM.

Description

[BasicTTIImpl] Fix getCastInstrCost for scalable vectors by querying for ElementCount.

This fixes an overly restrictive assumption that the vector is a FixedVectorType,
in code that tries to calculate the cost of a cast operation when splitting
a too-wide vector. The algorithm works the same for scalable vectors, so this
patch removes the cast<FixedVectorType>.

Reviewed By: david-arm

Differential Revision: https://reviews.llvm.org/D96253