arc: Update ctz/clz patterns

arc: Update ctz/clz patterns

ARCv2 ISA introduces special clz/ctz instructions. This patch is
adding support for them when available.

Corner case:
mov            r0,0x0 : (w0) r0 <= 0x00000000 *
ffs            r1,r0 : (w0) r1 <= 0x0000001f *
fls            r2,r0 : (w0) r2 <= 0x00000000 *

gcc/
2021-05-10  Claudiu Zissulescu  <claziss@synopsys.com>

	* config/arc/arc.h (CLZ_DEFINED_VALUE_AT_ZERO): Define.
	(CTZ_DEFINED_VALUE_AT_ZERO): Likewise.
	* config/arc/arc.md (clrsbsi2): Cleanup pattern.
	(norm_f): Likewise.
	(ffs): Likewise.
	(ffs_f): Likewise.
	(clzsi2): Use fls instruction when available.
	(arc_clzsi2): Likewise.

Signed-off-by: Claudiu Zissulescu <claziss@synopsys.com>