convert_to_complex vs invalid_conversion [PR111903]

convert_to_complex when creating a COMPLEX_EXPR does
not currently check if either the real or imag parts
was not error_mark_node. This later on confuses the gimpilfier
when there was a SAVE_EXPR wrapped around that COMPLEX_EXPR.
The simple fix is after calling convert inside convert_to_complex_1,
check that the either result was an error_operand and return
an error_mark_node in that case.

Bootstrapped and tested on x86_64-linux-gnu with no regressions.

	PR c/111903

gcc/ChangeLog:

	* convert.cc (convert_to_complex_1): Return
	error_mark_node if either convert was an error
	when converting from a scalar.

gcc/testsuite/ChangeLog:

	* gcc.target/i386/float16-8.c: New test.