[RISCV] Add patterns for fixed vector vwsll (#87316)

Fixed vectors have their sext/zext operands legalized to _VL nodes, so
we need to handle them in the patterns.

This adds a riscv_ext_vl_oneuse pattern since we don't care about the
type of extension used for the shift amount, and extends
Low8BitsSplatPat to handle other _VL nodes. We don't actually need to
check the mask or VL there since none of the _VL nodes have passthru
operands.

The remaining test cases that are widening from i8->i64 need to be
handled by extending combineBinOp_VLToVWBinOp_VL.

This also fixes Low8BitsSplatPat incorrectly checking the vector size
instead of the element size to determine if the splat value might have
been truncated below 8 bits.