RISC-V: Optimize slli(.uw)? + addw + zext.w into sh[123]add + zext.w

gcc/ChangeLog:

	* config/riscv/bitmanip.md: Handle corner-cases for combine
	when chaining slli(.uw)? + addw
	* config/riscv/riscv-protos.h (riscv_shamt_matches_mask_p):
	Define prototype.
	* config/riscv/riscv.cc (riscv_shamt_matches_mask_p):
	Helper for evaluating the relationship between two operands.

gcc/testsuite/ChangeLog:

	* gcc.target/riscv/zba-shNadd-04.c: New test.