[Bug target/53949] [SH] Add support for mac.w / mac.l instructions

["olegendo at gcc dot gnu.org" <gcc-bugzilla at gcc dot gnu dot org>]

http://gcc.gnu.org/bugzilla/show_bug.cgi?id=53949

--- Comment #11 from Oleg Endo <olegendo at gcc dot gnu.org> ---
Another question is whether the following is OK to do on all SH
implementations:

int test33 (int x, int y, int z)
{
  return x * y + z;
}

currently compiles:
        mul.l   r5,r4
        sts     macl,r0
        rts
        add     r6,r0

could also be done as:
        lds     r6,macl
        mov.l   r4,@-r15
        mov.l   r5,@-r15
        mac.l   @r15+,@r15+
        rts
        sts     macl,r0

This assumes that a mac insn with both address operands being the same works
exactly as it's described in the Renesas manuals:

 tempn = Read_32 (R[n]);
  R[n] += 4;
  tempm = Read_32 (R[m]);
  R[m] += 4;

However, I don't know whether this is true for all SH implementations.