[committed] [RISC-V] Provide splitting guidance to combine to faciliate shNadd.uw...

[committed] [RISC-V] Provide splitting guidance to combine to faciliate shNadd.uw generation

This fixes a minor code quality issue I found while comparing GCC and LLVM.
Essentially we want to do a bit of re-association to generate shNadd.uw
instructions.

Combine does the right thing and finds all the necessary instructions,
reassociates the operands, combines constants, etc.  Where is fails is finding
a good split point.  The backend can trivially provide guidance on how to split
via a define_split pattern.

This has survived both Ventana's internal CI system (rv64gcb) as well as my own
(rv64gc, rv32gcv).

I'll wait for the external CI system to give the all-clear before pushing.

gcc/
	* config/riscv/bitmanip.md: Add splitter for shadd feeding another
	add instruction.

gcc/testsuite/

	* gcc.target/riscv/zba-shadduw.c: New test.