CRIS: Simplify btst pattern slightly

The fix for PR rtl-optimization/126276 changed the mode of the result
of the post-split zero_extract for the *btst pattern from SImode to
that of the incoming (SImode, HImode, QImode) and required a slightly
more complex "btst" pattern.  That's not necessary: the zero_extract
is inherently zero-extending, so it's correct to only generate a
SImode result for the zero_extract in the define_insn_and_split split
insns, even if the incoming operand 0 is any of (SImode, HImode,
QImode).  So, the *btst pattern can be slightly less complex.

The comment removed in that commit at some time became stale, as well
as in the wrong place (post cc0 it should have been moved to the
recognizing parts of the generating define_insn_and_split:s).

	* config/cris/cris.md ("*btst<mode>): Simplify; iterate only over
	ZnNNZSET and compare only to a zero_extract result of SImode.
	Just add sanity checks for the mode of operand 0 in the condition.
	("*cbranch<mode>4_btstrq1_<CC>", "*cbranch<mode>4_btstqb0_<CC>"):
	Generate a zero_extract in SImode.