[SVE] Extend findMoreOptimalIndexType so BUILD_VECTORs do not force 64bit indices.

Extends findMoreOptimalIndexType to allow ISD::BUILD_VECTOR based
indices to be truncated when such truncation is lossless. This can
enable the use of 32bit gather/scatter indices thus making it less
likely to have to split a gather/scatter in two.

Depends on D125194

Differential Revision: https://reviews.llvm.org/D130533