RISC-V: Optimize masking with two clear bits not a SMALL_OPERAND

RISC-V: Optimize masking with two clear bits not a SMALL_OPERAND

Add a split for cases where we can use two bclri (or one bclri and an
andi) to clear two bits.

gcc/ChangeLog:

	* config/riscv/bitmanip.md (*bclri<mode>_nottwobits): New pattern.
	(*bclridisi_nottwobits): New pattern, handling the sign-bit.
	* config/riscv/predicates.md (const_nottwobits_operand):
	New predicate.

gcc/testsuite/ChangeLog:

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