[RISC-V] Fix costing of LO_SUM expressions

This is a rewrite of a patch originally from Xianmiao Qu.  Xianmiao
noticed that the costs we compute for LO_SUM expressions was incorrect.
Essentially we costed based solely on the first input to the LO_SUM.

In a LO_SUM, the first input is almost always going to be a REG and thus
isn't interesting.  The second argument is almost always going to be
some kind of symbolic operand, which is much more interesting from a
costing standpoint.

The right way to fix this is to sum the cost of the two operands.  I've
verified this produces the same code as Xianmiao's Qu's original patch.

This has been tested on rv32 and rv64 in my tester.  It missed today's
bootstrap of riscv64 though :(  Naturally I'll wait on the pre-commit CI
tester to render a verdict, but I don't expect any problems.

--  From Xianmiao Qu's original submission --

Currently, the cost of the LO_SUM expression is based on
the cost of calculating the first subexpression. When the
first subexpression is a register, the cost result will
be zero. It seems a bit unreasonable for a SET expression
to have a zero cost when its source is LO_SUM. Moreover,
having a cost of zero for the expression will lead the
loop invariant pass to calculate its benefits of being
moved outside the loop as zero, thus preventing the
out-of-loop placement of the loop invariant.

As an example, consider the following test case:
   long a;
   long b[];
   long *c;
   foo () {
     for (;;)
       *c = b[a];
   }

When compiling with -march=rv64gc -mabi=lp64d -Os, the following code is
generated:
         .cfi_startproc
         lui     a5,%hi(c)
         ld      a4,%lo(c)(a5)
         lui     a2,%hi(b)
         lui     a1,%hi(a)
.L2:
         ld      a5,%lo(a)(a1)
         addi    a3,a2,%lo(b)
         slli    a5,a5,3
         add     a5,a5,a3
         ld      a5,0(a5)
         sd      a5,0(a4)
         j       .L2

After adjust the cost of the LO_SUM expression, the instruction addi will be
moved outside the loop:
         .cfi_startproc
         lui     a5,%hi(c)
         ld      a3,%lo(c)(a5)
         lui     a4,%hi(b)
         lui     a2,%hi(a)
         addi    a4,a4,%lo(b)
.L2:
         ld      a5,%lo(a)(a2)
         slli    a5,a5,3
         add     a5,a5,a4
         ld      a5,0(a5)
         sd      a5,0(a3)
         j       .L2

gcc/
	* config/riscv/riscv.cc (riscv_rtx_costs): Correct costing of LO_SUM
	expressions.

Co-authored-by: Jeff Law <jlaw@ventanamicro.com>