[AArch64][GISel] Avoid scalarizing G_IMPLICIT_DEF and G_FREEZE in the Legalizer (#88469)

It does not make sense to scalarize G_FREEZE as it leads to the generation
of pairs of G_UNMERGE_VALUES and G_BUILD_VECTORs which are difficult to
optimize especially when operations like G_TRUNC operate before G_FREEZE
but after G_UNMERGE_VALUES.

Instead, it is better to legalize G_FREEZE like any other vector type
would be, as it gets lowered to a COPY during instruction selection
anyways.

This is an issue that was encountered when looking at the TSVC
benchmark, where the legalization of G_FREEZE would cause generation of
unnecessary MOVs that adversely affected the performance.