[ARM] Adjust default fp extend and trunc costs

This adds some default costs for fp extends and truncates, generally
costing them as 1 per lane. If the type is not legal then the cost will
include a call to an __aeabi_ function.

Some NEON code is also adjusted to make sure it applies to the expected
types, now that fp16 is a more common thing.

Differential Revision: https://reviews.llvm.org/D82458