[SVE] Fix casts to <FixedVectorType> in truncateToMinimalBitwidths

Fixes more casts to `<FixedVectorType>` for the cases where the
instruction is a Insert/ExtractElementInst.

For fixed-width, this part of truncateToMinimalBitWidths is tested by
AArch64/type-shrinkage-insertelt.ll. I attempted to write a test case for this part
of truncateToMinimalBitWidths which uses scalable vectors, but was unable to add
one. The tests in type-shrinkage-insertelt.ll rely on scalarization to create extract
element instructions for instance, which is not possible for scalable vectors.

Reviewed By: david-arm

Differential Revision: https://reviews.llvm.org/D106163