RISC-V/testsuite: Add branched cases for FP NE cond-add operation

RISC-V/testsuite: Add branched cases for FP NE cond-add operation

Verify, for the generic floating-point NE conditional-add operation,
that if-conversion does *not* trigger at `-mbranch-cost=2' setting,
which makes original branched code sequences cheaper than their
branchless equivalents if-conversion would emit.

	gcc/testsuite/
	* gcc.target/riscv/adddibfne.c: New test.
	* gcc.target/riscv/addsibfne.c: New test.