[RISC-V][PR target/118410] Improve code generation for some logical ops

I'm posting this on behalf of Shreya Munnangi who is working as an intern with
me.  I've got her digging into prerequisites for removing mvconst_internal and
would prefer she focus on that rather than our patch process at this time.

--

We can use the orn, xnor, andn instructions on RISC-V to improve the code
generated logical operations when one operand is a constant C where
synthesizing ~C is cheaper than synthesizing C.

This is going to be an N -> N - 1 splitter rather than a define_insn_and_split.
A define_insn_and_split can obviously work, but has multiple undesirable
effects in general.

As a result of implementing as a simple define_split we're not supporting AND
at this time.  We need to clean up the mvconst_internal situation first after
which supporting AND is trivial.

This has been tested in Ventana's CI system as well as my tester. Obviously
we'll wait for the pre-commit tester to run before moving forward.

	PR target/118410
gcc/
	* config/riscv/bitmanip.md (logical with constant argument): New
	splitter for cases where synthesizing ~C is cheaper than synthesizing
	the original constant C.

gcc/testsuite/
	* gcc.target/riscv/pr118410-1.c: New test.
	* gcc.target/riscv/pr118410-2.c: Likewise.

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

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