vect: Handle demoting FLOAT and promoting FIX_TRUNC.

The recent changes that allowed multi-step conversions for
"non-packing/unpacking", i.e. modifier == NONE targets included
promoting to-float and demoting to-int variants.  This patch
adds the missing demoting to-float and promoting to-int handling.

gcc/ChangeLog:

	* tree-vect-stmts.cc (vectorizable_conversion): Handle
	more demotion/promotion for modifier == NONE.

gcc/testsuite/ChangeLog:

	* gcc.target/riscv/rvv/autovec/conversions/vec-narrow-int64-float16.c: New test.
	* gcc.target/riscv/rvv/autovec/conversions/vec-widen-float16-int64.c: New test.