[GlobalISel] Handle non-multiples of the base type in narrowScalarInsert

When narrowing G_INSERT, handle types that aren't a multiple of the
type we're narrowing to. This comes up if we're narrowing something
like an s96 to fit in 64 bit registers and also for non-byte multiple
packed types if they come up.

This implementation handles these cases by extending the extra bits to
the narrow size and truncating the result back to the destination
size.

Differential Revision: https://reviews.llvm.org/D97791