[AArch64][GlobalISel] Legalize wide vector G_PHIs

Clamp the max number of elements when legalizing G_PHI. This allows us to
legalize some common fallbacks like 4 x s64.

Here's an example: https://godbolt.org/z/6YocsEYTd

Had to add -global-isel-abort=0 to legalize-phi.mir to account for the
G_EXTRACT_VECTOR_ELT from the 32 x s8 G_PHI.

Differential Revision: https://reviews.llvm.org/D107508