ARC: Convert (signed<<31)>>31 to -(signed&1) without barrel shifter.

ARC: Convert (signed<<31)>>31 to -(signed&1) without barrel shifter.

This patch optimizes PR middle-end/101955 for the ARC backend.  On ARC
CPUs with a barrel shifter, using two shifts is optimal as:

        asl_s   r0,r0,31
        asr_s   r0,r0,31

but without a barrel shifter, GCC -O2 -mcpu=em currently generates:

        and     r2,r0,1
        ror     r2,r2
        add.f   0,r2,r2
        sbc     r0,r0,r0

with this patch, we now generate the smaller, faster and non-flags
clobbering:

        bmsk_s  r0,r0,0
        neg_s   r0,r0

2023-10-30  Roger Sayle  <roger@nextmovesoftware.com>

gcc/ChangeLog
	PR middle-end/101955
	* config/arc/arc.md (*extvsi_1_0): New define_insn_and_split
	to convert sign extract of the least significant bit into an
	AND $1 then a NEG when !TARGET_BARREL_SHIFTER.

gcc/testsuite/ChangeLog
	PR middle-end/101955
	* gcc.target/arc/pr101955.c: New test case.