[SVE][CodeGen] Improve codegen for some FP insert_subvector cases

When inserting an unpacked FP subvector into a packed vector we
can simply cast the unpacked value into a packed value, since
both types are legal for SVE. We can then use this as the input
for the UZP instruction. This avoids us expanding the operation
by going through the stack.

Differential Revision: https://reviews.llvm.org/D113270