analyzer: update "tainted" state of RHS in comparisons [PR106373]

analyzer: update "tainted" state of RHS in comparisons [PR106373]

Doing so fixes various false positives from
-Wanalyzer-tainted-array-index at -O1 and above (e.g. seen on the
Linux kernel)

gcc/analyzer/ChangeLog:
	PR analyzer/106373
	* sm-taint.cc (taint_state_machine::on_condition): Potentially
	update the state of the RHS as well as the LHS.

gcc/testsuite/ChangeLog:
	PR analyzer/106373
	* gcc.dg/analyzer/torture/taint-read-index-3.c: New test.

Signed-off-by: David Malcolm <dmalcolm@redhat.com>