Fix 624-checker-stringops for PIC.

Do not rely on the const-string "x" to be non-throwing;
just pull the "x" out of the loop to make it irrelevant.
(The non-PIC test was "lucky" because "x" is a boot image
string and HLoadString/kBootImageAddress in non-throwing.
In PIC mode, the HLoadString/kBssEntry is throwing; the
"is in boot image" optimization for HLoadClass has not been
implemented for HLoadString.)

Test: testrunner.py --host -t 624
Test: testrunner.py --host --pictest -t 624
Change-Id: Iff5cfb1276af0e4896707f19a18e6053afd87a77