[RISC-V] Avoid multiple assignments to output object

[RISC-V] Avoid multiple assignments to output object

This is the next batch of changes to reduce multiple assignments to an output
object.  This time I'm focused on splitters in bitmanip.md.

This doesn't convert every case.  For example there is one case that is very
clearly dependent on eliminating mvconst_internal and adjustment of a splitter
for andn and until those things happen it would clearly be a QOI implementation
regression.

There are cases where we set a scratch register more than once.  It may be
possible to use an additional scratch.  I haven't tried that yet.

I've seen one failure to if-convert a sequence after this patch, but it should
be resolved once the logical AND changes are merged.  Otherwise I'm primarily
seeing slight differences in register allocation and scheduling.  Nothing
concerning to me.

This has run through my tester, but I obviously want to see how it behaves in
the upstream CI system as that tests slightly different multilibs than mine (on
purpose).

gcc/

	* config/riscv/bitmanip.md (various splits): Avoid writing the output
	more than once when trivially possible.