[AArch64][GlobalISel] Select UMULL instruction (#65469)

Global ISel now selects `UMULL` and `UMULL2` instructions.
G_MUL instruction with input operands coming from `SEXT` or `ZEXT`
operations are turned into UMULL

G_MUL instructions with v2s64 result type is always scalarised except: 
`mul ( unmerge( ext ), unmerge( ext ))` 

So the extend could be unmerged and fold away the unmerge in the middle:
`mul ( unmerge( ext ), unmerge( ext ))` =>
`mul ( unmerge( merge( ext( unmerge )), unmerge( merge( ext( unmerge
))))` =>
`mul ( ext(unmerge)), ( ext( unmerge ))) `