RISC-V: allow bseti on SImode without sign-extension

As long as the SImode operand is not a partial subreg, we can use a
bseti without postprocessing to or in a bit, as the middle end is
smart enough to stay away from the signbit.

gcc/ChangeLog:

	* config/riscv/bitmanip.md (*bsetidisi): New pattern.

gcc/testsuite/ChangeLog:

	* gcc.target/riscv/zbs-bseti-02.c: New test.