[PATCH v2] RISC-V: Fix scalar floating-point costs

Scalar IL costing receives a null vectype, causing floating-point
statements to use the integer cost. Recover the scalar type and restrict
LMUL adjustments to vector types.

gcc/ChangeLog:

	* config/riscv/riscv-vector-costs.cc (costs::adjust_stmt_cost):
	Restrict LMUL scaling to vector types.
	(costs::add_stmt_cost): Recover the scalar type for scalar costing.

gcc/testsuite/ChangeLog:

	* gcc.target/riscv/rvv/autovec/scalar-fp-cost.c: New test.