Fix testsuite scan failure on loongarch64

Fix testsuite scan failure on loongarch64

Robin's patch to fix nonzero bits for the result of a popcount triggered a
minor testsuite scan failure for loongarch.  Essentially it was expecting to
see an ANDI for a zero-extend, but after the change we get a slli with a count
0, which is used for sign extension on loongarch.

Both are likely equally performant.  So this just adjusts the test to accept
both forms.  Now one could argue that the result of the popcount is already
sign extended and you'd be right -- that's a separate missed optimization issue
and unrelated to this testsuite regression.

Anyway, pushing this to the trunk.

gcc/testsuite/
	* gcc.dg/pr90838.c: Adjust expected output for loongarch.