The expression that caused the ICE

	++(a += 1.0);

The D front-end rewrites and applies implicit type conversions so the
expression gets simplified as

	(int)((double) a += 1.0) += 1

The codegen pass would subsequently generate the following invalid code

	(int)(double) a = (int)((double) a + 1.0) + 1

The LHS expression `(int)(double) a', represented as a FIX_TRUNC_EXPR
being what trips as it is not a valid lvalue for assignment.

While LHS casts are stripped away, convert_expr adds a double cast
because it converts the expression to its original type before
converting it to its target type.  There is no valid reason why this is
done, so it has been removed.

gcc/d/ChangeLog:

	PR d/90601
	* d-convert.cc (convert_expr): Don't convert an expression to its
	original front-end type before converting to its target type.

gcc/testsuite/ChangeLog:

	PR d/90601
	* gdc.dg/pr90601.d: New test.

From-SVN: r274263