We want to use vfp_expand_imm() in the AArch32 VFP decode;
move it from the a64-only header/source file to the
AArch32 one (which is always compiled even for AArch64).
Backports commit d6a092d479333b5f20a647a912a31b0102d37335 from qemu
Rearrange the arithmetic so that we are agnostic about the total size
of the vector and the size of the element. This will allow us to index
up to the 32nd byte and with 16-byte elements.
Backports commit 66f2dbd783d0b6172043e3679171421b2d0bac11 from qemu
