Previously, only 128-bit "NEON" vectors were given sensible costs.
Cores with vscale>1 can use SVE's EXT instruction to perform a
fixed-length subvector extract.
This is a follow-up from the codegen patches at #152554. They show that
with the help of MOVPRFX, we can do subvector extracts with roughly one
instruction. We now at least give sensible costs for extracting 128-bit
halves from a 256-bit vector.
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