[SelectionDAGBuilder] use bitcast instead of AnyExtOrTrunc if copy parts from...

[SelectionDAGBuilder] use bitcast instead of AnyExtOrTrunc if copy parts from an int vector to a float vector to fix issue #58615

The getCopyFromPartsVector doesn't work correctly when PartEVT and ValueVT have both different element type and different size.

This patch
1) removes the part of a comment that contains the incorrect assumption that element type are the same
2) use bitcast when copy parts of int vector to a float vector after the subvector extraction

Reviewed By: Peter, efriedma

Differential Revision: https://reviews.llvm.org/D136726