[SVE] Don't require lookup when demangling vector function mappings (#72260)

We can determine the VF from a combination of the mangled name (which
indicates the arguments that take vectors) and the element sizes of
the arguments for the scalar function the mapping has been established
for.

The assert when demangling fails has been removed in favour of just
not adding the mapping, which prevents the crash seen in
https://github.com/llvm/llvm-project/issues/71892

This patch also stops using _LLVM_ as an ISA for scalable vector tests,
since there aren't defined rules for the way vector arguments should be
handled (e.g. packed vs. unpacked representation).