[RISCV] Shrink vslidedown when lowering fixed extract_subvector (#65598)

As noted in
https://github.com/llvm/llvm-project/pull/65392#discussion_r1316259471,
when lowering an extract of a fixed length vector from another vector,
we don't need to perform the vslidedown on the full vector type. Instead
we can extract the smallest subregister that contains the subvector to
be extracted and perform the vslidedown with a smaller LMUL. E.g, with
+Zvl128b:

v2i64 = extract_subvector nxv4i64, 2

is currently lowered as

vsetivli zero, 2, e64, m4, ta, ma
vslidedown.vi v8, v8, 2

This patch shrinks the vslidedown to LMUL=2:

vsetivli zero, 2, e64, m2, ta, ma
vslidedown.vi v8, v8, 2

Because we know that there's at least 128*2=256 bits in v8 at LMUL=2,
and we only need the first 256 bits to extract a v2i64 at index 2.

lowerEXTRACT_VECTOR_ELT already has this logic, so this extracts it out
and reuses it.

I've split this out into a separate PR rather than include it in #65392,
with the hope that we'll be able to generalize it later.

This patch refactors extract_subvector lowering to lower to
extract_subreg directly, and to shortcut whenever the index is 0 when
extracting a scalable vector. This doesn't change any of the existing
behaviour, but makes an upcoming patch that extends the scalable path
slightly easier to read.