RISC-V/testsuite: Add branchless cases for generic integer cond adds

Verify, for generic integer conditional-add operations, if-conversion
to trigger via `noce_try_addcc' at the respective sufficiently high
`-mbranch-cost=' settings that make branchless code sequences produced
by if-conversion cheaper than their original branched equivalents, and,
where applicable, that extraneous instructions such as SNEZ, etc. are
not present in output.  Cover all integer relational operations to make
sure no corner case escapes.

The reason to XFAIL SImode tests for RV64 targets is the compiler thinks
it has to sign-extend addends, which causes if-conversion to give up.

	gcc/testsuite/
	* gcc.target/riscv/adddieq.c: New test.
	* gcc.target/riscv/adddige.c: New test.
	* gcc.target/riscv/adddigeu.c: New test.
	* gcc.target/riscv/adddigt.c: New test.
	* gcc.target/riscv/adddigtu.c: New test.
	* gcc.target/riscv/adddile.c: New test.
	* gcc.target/riscv/adddileu.c: New test.
	* gcc.target/riscv/adddilt.c: New test.
	* gcc.target/riscv/adddiltu.c: New test.
	* gcc.target/riscv/adddine.c: New test.
	* gcc.target/riscv/addsieq.c: New test.
	* gcc.target/riscv/addsige.c: New test.
	* gcc.target/riscv/addsigeu.c: New test.
	* gcc.target/riscv/addsigt.c: New test.
	* gcc.target/riscv/addsigtu.c: New test.
	* gcc.target/riscv/addsile.c: New test.
	* gcc.target/riscv/addsileu.c: New test.
	* gcc.target/riscv/addsilt.c: New test.
	* gcc.target/riscv/addsiltu.c: New test.
	* gcc.target/riscv/addsine.c: New test.