match.pd: Use element_mode instead of TYPE_MODE.

match.pd: Use element_mode instead of TYPE_MODE.

This patch changes TYPE_MODE into element_mode in a match.pd
simplification.  As the simplification can be also called with vector
types real_can_shorten_arithmetic would ICE in REAL_MODE_FORMAT which
expects a scalar mode.  Therefore, use element_mode instead of
TYPE_MODE.

Additionally, check if the target supports the resulting operation.  One
target that supports e.g. a float addition but not a _Float16 addition
is the RISC-V vector extension Zvfhmin.

gcc/ChangeLog:

	* match.pd: Use element_mode and check if target supports
	operation with new type.