rs6000: clz/ctz/ffs improvement (PR78683)

rs6000: clz/ctz/ffs improvement (PR78683)

On CPUs that implement popcnt[wd] but not cnttz[wd] we can do better for
the ctz sequences than we do today.

CL[TZ]_DEFINED_VALUE_AT_ZERO can return 2, since we always return the
same fixed value (only dependent on TARGET_* options).

	PR target/78683
	* config/rs6000/rs6000.h (CLZ_DEFINED_VALUE_AT_ZERO): Use
	GET_MODE_BITSIZE.  Return 2.
	(CTZ_DEFINED_VALUE_AT_ZERO): Use GET_MODE_BITSIZE.  Return 2.  Handle
	TARGET_POPCNTD the same as TARGET_CTZ.
	* config/rs6000/rs6000.md (ctz<mode>2): Reimplement.
	(ffs<mode>2): Reimplement.

From-SVN: r243499