Use different operations to load string before loops in LICM test.

The test assertion will not hold if a compiler (D8) removes the
string load, e.g., by computing String#length at compile time:
  https://r8-review.googlesource.com/c/r8/+/26540

Bug: 115968045
Bug: 113880427
Test: art/test.py --host -r -t 624-checker-stringops

Change-Id: Ibe6a4a46f534b7079ba516f27da82fec2d60f4ea