[X86] Improve the type checking fast-isel handling of vector bitcasts.

We had a bunch of vector size legality checks for the source type
based on feature flags, but we didn't check the destination type at
all beyond ensuring that it was a "simple" type. But this allowed
the destination to be i128 which isn't legal.

This commit changes the code to use TLI's isTypeLegal logic in
place of the all the subtarget checks. Then additionally checks
that the source and dest are vectors.

Fixes 42452

llvm-svn: 364729