middle-end/27800 - avoid unnecessary temporary during gimplification

middle-end/27800 - avoid unnecessary temporary during gimplification

This avoids a tempoary when gimplifying reg = a ? b : c, re-using
the LHS of an assignment if that's a register.

	PR middle-end/27800
	* gimplify.cc (gimplify_modify_expr_rhs): For a COND_EXPR
	avoid a temporary from gimplify_cond_expr when the LHS is
	a register by pushing the assignment into the COND_EXPR arms.

	* gcc.dg/pr27800.c: New testcase.