[RISCV] Set getMinVectorRegisterBitWidth to 16 if enable fixed length vector code gen for RVV

getMinVectorRegisterBitWidth means what vector types is supported in
this target, and actually RISC-V support all fixed length vector types with
vector length less than `getMinRVVVectorSizeInBits`, so set it to 16,
means 2 x i8, that is minimal fixed length vector size in theory.

That also fixed one issue, some testcase migth become non-vectorizable
when `-riscv-v-vector-bits-min` set to larger value, because the vector size is
smaller than `-riscv-v-vector-bits-min`.

For example, following code can vectorize by SLP with
`-riscv-v-vector-bits-min=128` or `-riscv-v-vector-bits-min=256`, but
can't vectorize `-riscv-v-vector-bits-min=512` or larger:

```
void foo(double *da) {
  da[0] = 0;
  da[1] = 1;
  da[2] = 2;
  da[3] = 3;
}
```

Reviewed By: craig.topper

Differential Revision: https://reviews.llvm.org/D116534