[ARM] Split large widening MVE loads

Similar to rL372717, we can force the splitting of extends of vector loads in
MVE, in order to use the better widening loads as opposed to going through
expensive extends. This adds a combine to early-on detect extends of loads and
split the load in two, from where normal legalisation will kick in and we get a
series of widening loads.

Differential Revision: https://reviews.llvm.org/D67909

llvm-svn: 372721