combine: Convert subreg-of-lshiftrt to zero_extract properly (PR78390)

combine: Convert subreg-of-lshiftrt to zero_extract properly (PR78390)

r242414, for PR77881, introduces some bugs (PR78390, PR78438, PR78477).
It all has the same root cause: that patch makes combine convert every
lowpart subreg of a logical shift right to a zero_extract.  This cannot
work at all if it is not a constant shift, and it has to be a bit more
careful exactly which bits it extracts.

	PR target/77881
	PR bootstrap/78390
	PR target/78438
	PR bootstrap/78477
	* combine.c (make_compound_operation_int): Do not convert a subreg of
	a non-constant logical shift right to a zero_extract.  Handle the case
	where some zero bits have been shifted into the range covered by that
	subreg.

From-SVN: r242757