[AArch64] Add fpext and fpround costs (#119292)
This adds some basic costs for fpext and fpround, many of which were
already handled by the generic costing routines but this does make some
adjustments for larger vector types that can use fcvtn+fcvtn2, as
opposed to fcvtn+fcvtn+concat.
These should now more closely match the codegen from
https://godbolt.org/z/r3P9Mf8ez, for example.
