c++: Avoid some false positive -Wfloat-conversion warnings with extended precision...

c++: Avoid some false positive -Wfloat-conversion warnings with extended precision [PR108285]

On the following testcase trunk emits a false positive warning on ia32.
convert_like_internal is there called with type of double and
expr EXCESS_PRECISION_EXPR with float type with long double operand
2.L * (long double) x.
Now, for the code generation we do the right thing, cp_convert
to double from that 2.L * (long double) x, but we call even
cp_convert_and_check with that and that emits the -Wfloat-conversion
warning.  Looking at what the C FE does in this case, it calls
convert_and_check with the EXCESS_PRECISION_EXPR expression rather
than its operand, and essentially uses the operand for code generation
and EXCESS_PRECISION_EXPR itself for warnings.

The following patch does that too for the C++ FE.

2023-01-13  Jakub Jelinek  <jakub@redhat.com>

	PR c++/108285
	* cvt.cc (cp_convert_and_check): For EXCESS_PRECISION_EXPR
	use its operand except that for warning purposes use the original
	EXCESS_PRECISION_EXPR.
	* call.cc (convert_like_internal): Only look through
	EXCESS_PRECISION_EXPR when calling cp_convert, not when calling
	cp_convert_and_check.

	* g++.dg/warn/pr108285.C: New test.