[AArch64] Handle 64bit vectors in tryCombineFixedPointConvert

Under some situations we can visit 64bit vector extract elements in
tryCombineFixedPointConvert, where an assert fires as they are expected
to have been converted to 128bit. Turn the assert into an if statement,
bailing out and letting the extract be handled first.

Also invert some ifs, using early exits to reduce indentation.

Fixes #55417