[Bug target/109279] RISC-V: complex constants synthesized should be improved

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

https://gcc.gnu.org/bugzilla/show_bug.cgi?id=109279

--- Comment #17 from Vineet Gupta <vineetg at gcc dot gnu.org> ---
(In reply to Vineet Gupta from comment #16)
> > Which is what this produces:
> > ```
> > long long f(void)
> > {
> >   unsigned t = 16843009;
> >   long long t1 = t;
> >   long long t2 = ((unsigned long long )t) << 32;
> >   asm("":"+r"(t1));
> >   return t1 | t2;
> > }
> > ```

> 
> 	li	a0,16842752
> 	addi	a0,a0,257
> 	li	a5,16842752
> 	slli	a0,a0,32
> 	addi	a5,a5,257
> 	or	a0,a5,a0
> 	ret

This is again IRA inflicted pain (similar to [PR110748]). 
IRA seems to be undoing split1 since we have 2 insn sequences to synthesize the
constant pieces. This explains why the problem got exacerbated with commit
0530254413f8 ("riscv: relax splitter restrictions for creating pseudos") since
now different regs are used to create parts of const, vs 1 reg being repeatedly
used for assembling a const (fooling IRA's equivalent replacement logic).