RISC-V: bitmanip: add splitter to use bexti for "(a & (1 << BIT_NO)) ? 0 : -1"

Consider creating a polarity-reversed mask from a set-bit (i.e., if
the bit is set, produce all-ones; otherwise: all-zeros).  Using Zbb,
this can be expressed as bexti, followed by an addi of minus-one.  To
enable the combiner to discover this opportunity, we need to split the
canonical expression for "(a & (1 << BIT_NO)) ? 0 : -1" into a form
combinable into bexti.

Consider the function:
    long f(long a)
    {
      return (a & (1 << BIT_NO)) ? 0 : -1;
    }
This produces the following sequence prior to this change:
	andi	a0,a0,16
	seqz	a0,a0
	neg	a0,a0
	ret
Following this change, it results in:
	bexti	a0,a0,4
	addi	a0,a0,-1
	ret

gcc/ChangeLog:

	* config/riscv/bitmanip.md: Add a splitter to generate
	  polarity-reversed masks from a set bit using bexti + addi.

gcc/testsuite/ChangeLog:

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