RISC-V: Replace not + bitwise_imm with li + bitwise_not

In the case when we have C code like this

int foo (int a) {
   return 100 & ~a;
}

GCC generates the following instruction sequence

foo:
     not     a0,a0
     andi    a0,a0,100
     ret

This patch replaces that with this sequence
foo:
     li a5,100
     andn a0,a5,a0
     ret

The profitability comes from an out-of-order processor being able to
issue the "li a5, 100" at any time after it's fetched while "not a0, a0" has
to wait until any prior setter of a0 has reached completion.

gcc/ChangeLog:
	* config/riscv/bitmanip.md (*<optab>_not_const<mode>): New split
	pattern.

gcc/testsuite/ChangeLog:
	* gcc.target/riscv/zbb-andn-orn-01.c: New test.
	* gcc.target/riscv/zbb-andn-orn-02.c: Likewise.