[AggressiveInstCombine] Fix invalid TypeSize conversion when combining loads.

Much of foldLoadsRecursive relies on knowing the size of loaded
data, which is not possible for scalable vector types.  However,
the logic of combining two small loads into one bigger load does
not apply for vector types so rather than converting the algorithm
to use TypeSize I've simply added an early exit for vectors.

Fixes #59510

Differential Revision: https://reviews.llvm.org/D140106