[RISCV] Match strided vector bases in RISCVGatherScatterLowering (#93972)

Currently we only match GEPs with a scalar base pointer, but a common
pattern that's emitted from the loop vectorizer is a strided vector base
plus some sort of scalar offset:

    %base = getelementptr i64, ptr %p, <vscale x 1 x i64> %step
    %gep = getelementptr i64, <vscale x 1 x ptr> %base, i64 %offset

This is common for accesses into a struct e.g. f[i].b below:

    struct F { int a; char b; };

    void foo(struct F *f) {
      for (int i = 0; i < 1024; i += 2) {
        f[i].a++;
        f[i].b++;
      }
    }

This patch handles this case in RISCVGatherScatterLowering by recursing
on the base pointer if it's a vector.

With this we can convert roughly 80% of the indexed loads and stores
emitted to strided loads and stores on SPEC CPU 2017, -O3
-march=rva22u64_v