[CodeGen][AArch64] Fix isel crash for truncating FP stores

When attempting to truncate a FP vector and store the result out
to memory we crashed because we had no pattern for truncating FP
stores. In fact, we don't support these types of stores and the
correct fix is to stop marking these truncating stores as legal.

Tests have been added here:

  CodeGen/AArch64/sve-fptrunc-store.ll

Differential Revision: https://reviews.llvm.org/D100025