d: Suboptimal codegen for __builtin_expect(cond, false)

Since PR96435, both boolean objects and expressions have been evaluated
in the following way.

    (*(ubyte*)&obj_or_expr) & 1

It has been noted that sometimes this can cause the back-end to optimize
in non-obvious ways - in particular with __builtin_expect.

This @safe feature is now restricted to just when reading the value of a
bool field that comes from a union.

	PR d/110359

gcc/d/ChangeLog:

	* d-convert.cc (convert_for_rvalue): Only apply the @safe boolean
	conversion to boolean fields of a union.
	(convert_for_condition): Call convert_for_rvalue in the default case.

gcc/testsuite/ChangeLog:

	* gdc.dg/pr110359.d: New test.