[AArch64][GlobalISel] Fall back if disabling neon/fp in the translator.

The previous technique relied on early-exiting the legalizer predicate
initialization, leaving an empty rule table. That causes a fallback
for most instructions, but some have legacy rules defined like G_ZEXT
which can try continue, but then crash.

We should fall back earlier, in the translator, to avoid this issue.

Differential Revision: https://reviews.llvm.org/D98730