[ARM] Split FPExt loads

This extends PerformSplittingToWideningLoad to also handle FP_Ext, as
well as sign and zero extends. It uses an integer extending load
followed by a VCVTL on the bottom lanes to efficiently perform an fpext
on a smaller than legal type.

The existing code had to be rewritten a little to not just split the
node in two and let legalization handle it from there, but to actually
split into legal chunks.

Differential Revision: https://reviews.llvm.org/D81340