[AArch64][GlobalISel] Lower scalarizing G_UNMERGE_VALUES to G_EXTRACT_VECTOR_ELT

This adds post-legalizing lowering of G_UNMERGE_VALUES which take a vector and
produce scalar values for each lane. They are converted to a G_EXTRACT_VECTOR_ELT
for each lane, allowing all the existing tablegen patterns to apply to them.

A couple of tablegen patterns need to be altered to make sure the type of the
constant operand is known, so that the patterns are recognized under global
isel.

Closes #75662

GitOrigin-RevId: c0931d4950a93526aa08ec3ab86f64ffb616b406