[AArch64][GlobalISel] Avoid splitting loads of large vector types into...

[AArch64][GlobalISel] Avoid splitting loads of large vector types into individual element loads (#85042)

This patch fixes an issue with the legalization of G_LOAD where the
presence of .lowerIfMemSizeNotByteSizePow2 before .clampMaxNumElements
was causing issues for vectors which matched that condition.

Such vectors would be lowered into per-element loads instead of being
split up into 128-bit chunks.