middle-end/115426 - wrong gimplification of "rm" asm output operand

When the operand is gimplified to an extract of a register or a
register we have to disallow memory as we otherwise fail to
gimplify it properly.  Instead of

  __asm__("" : "=rm" __imag <r>);

we want

  __asm__("" : "=rm" D.2772);
  _1 = REALPART_EXPR <r>;
  r = COMPLEX_EXPR <_1, D.2772>;

otherwise SSA rewrite will fail and generate wrong code with 'r'
left bare in the asm output.

	PR middle-end/115426
	* gimplify.cc (gimplify_asm_expr): Handle "rm" output
	constraint gimplified to a register (operation).

	* gcc.dg/pr115426.c: New testcase.