vect: Allow same precision for bit-precision conversions.

In PR111794 we miss a vectorization because on riscv type precision and
mode precision differ for mask types.  We can still vectorize when
allowing assignments with the same precision for dest and source which
is what this patch does.

gcc/ChangeLog:

	PR tree-optimization/111794
	* tree-vect-stmts.cc (vectorizable_assignment): Add
	same-precision exception for dest and source.

gcc/testsuite/ChangeLog:

	* gcc.target/riscv/rvv/autovec/slp-mask-1.c: New test.
	* gcc.target/riscv/rvv/autovec/slp-mask-run-1.c: New test.